BUG: resample fixes
make sure .resample(...).plot() warns and returns a correct plotting object
make sure that .groupby(...).resample(....) is hitting warnings when appropriate

closes #12448

@jorisvandenbossche this should take care of 1) and 3) in #12448 and doc 2)
 @jorisvandenbossche ?
 It is possible that the previous workaround with warning ("resample is now a deferred ..") for accessing elements does not work anymore?

```
In [13]: s.resample('15min').mean()[0]
Out[13]: -0.27773169409289944

In [14]: s.resample('15min')[0]
KeyError: 'Column not found: 0'
```

I thought this last one worked (gave the same result), but warned?
 For case 3, I still not get the same as in 0.17.1. With this PR:

```
In [19]: df.groupby('group').resample('1D', fill_method='ffill')
C:\Anaconda\envs\devel\Scripts\ipython-script.py:1: FutureWarning: fill_method i
s deprecated to .resample()
the new syntax is .resample(...).ffill()
  if __name__ == '__main__':
Out[19]:
            val
date
2016-01-03    5
2016-01-10    6
2016-01-17    7
2016-01-24    8
```

So the actual grouper has disappeared
 You get the same `KeyError` in 0.17.1 when you are using a `DataFrame`

```
In [5]: s.to_frame('foo').resample('15T')[0]
```

I will fix the `Series` behavior
 @jorisvandenbossche 

see #12486 

I can't fix this now. I _think_ its possible, but will require some work. This is NOT the same as a multi-groupby of a column / freq, because resampling does some auto-filling. To be honest this is prob a bit to magical in the past.
 @jorisvandenbossche any comments?
 The error with the Series is still there (this is not necessarily related to the changes in this PR though):

```
In [21]: s.resample('2s').mean()[0]
Out[21]: 0.22864360896621477

In [22]: s.resample('2s')[0]
KeyError: 'Column not found: 0'
```

I think this should give something like iloc does?:

```
In [23]: s.resample('2s').iloc[0]
ValueError: .resample() is now a deferred operation
        use .resample(...).mean() instead of .resample(...)
        assignment will have no effect as you are working on a copy
```
 BTW, the _"assignment will have no effect as you are working on a copy"_ in that error message is not really clear to me. It's not that it has no effect, as it raises an error.
 On the PR itself, plot change and whatsnew update looks good!
 Something else with getitem:

```
In [8]: rs['2010-01-01 09:00:00']

AbstractMethodError: This method must be defined in the concrete class of Series
GroupBy
```

But these getitem things are not really related to this PR, so I can open another issue for that, and then this PR can be merged
 Are you sure you did with this PR? (for the getitem issue). This is explicity fixed/mentioned.

```
In [5]: s.resample('2s').mean()[0]
Out[5]: -0.0026492975424703812

In [6]: s.resample('2s')[0]
Out[6]: -0.0026492975424703812
```
 took out the assignment part of the message 
 ok, both those errors fixed (and now they show the deprecation warning as well).

ping if ok and i'll merge (I need to squash)
 Looks good!
 ok!
 Hi,

I have a FutureWarning from pandas 0.18.0 related to a `resample` operation which seems inappropriate:

``` python
import pandas as pd
import datetime as dt

df=pd.DataFrame(data=[1,3], index=[dt.timedelta(), dt.timedelta(minutes=3)])
df.resample('1T').interpolate(method='linear')
```

The result I get from the last line is correct but I also get the following warning:

```
FutureWarning: .resample() is now a deferred operation use .resample(...).mean() instead of .resample(...)
```

What is wrong with my syntax?
Thanks
 @benoit9126 No, I _think_ this warning is correct, and I think what you want is: 

```
df.resample('1T').mean().interpolate(method='linear')
```

`interpolate` is not a method that is available on a Resample object. You first have to indicate how you want to resample the values (in this case using `mean` does work because you downsample).

But that aside, it does maybe make sense to have `interpolate` available, as this can work similarly to to `resample().fillna()` 
 @benoit9126 as @jorisvandenbossche points out, in < 0.18.0 there was an implicit `how=mean` being done when you said `df.resample('1T)` w/o any `how=` arg. 

@jorisvandenbossche yes, having a `.interpolate` method would be fine (and very straightforward to do actually). if you can open an issue with a nice example would be great.
 @jreback Something else I just noticed, seems like a bug in `asfreq`, but didn't yet test with master:

```
In [16]: df=pd.DataFrame(data=[1,3], index=[dt.timedelta(), dt.timedelta(minutes
=3)])
In [17]: df
Out[17]:
          0
00:00:00  1
00:03:00  3

In [18]: df.resample('1T').mean()
Out[18]:
            0
00:00:00  1.0
00:01:00  NaN
00:02:00  NaN
00:03:00  3.0

In [19]: df.resample('1T').asfreq()
Out[19]:
            0
00:00:00  1.0
00:01:00  NaN
00:02:00  NaN
```

Missing the the last value in asfreq?
 @jorisvandenbossche hmm, that last does look like a bug, can you create an issue and i'll take a look. ty.
 Thanks a lot for the explanation.
Nevertheless, I keep my code as it is hoping that it won't throw a warning in pandas 0.18.2 :wink: 
 @benoit9126 To be clear, there is no guarantee that #12925 will be implemented by 0.19, so it is possible that this code will start raising an error. Your responsibility :-) (or always welcome to put up a PR! I don't think it will be that hard to implement)
 @jorisvandenbossche To be honest I do not have enough time to implement this feature in a close future (especially because I have never carefully inspected the pandas core). If it changes, it will be a pleasure.
