ENH: Scatter plot now supports errorbar
This is enhancement for #5638. I sometimes want to plot scatter with errorbars, thus I've refactored to support it.

![figure_1](https://cloud.githubusercontent.com/assets/1696302/2632517/21edcfd6-be65-11e3-9cd9-97bad65c8129.png)

Also, this includes following fixes. I've also modified tests to check the number of errorbars actually drawn.
- There are some types which expected to work, but results in `ValueError`.
  - `float` (MPL can accept it)
  - `unicode` (in Python 2.x)
  - Other list-like, such as `iterator`
- A bug related to time series.

```
>>> s = Series(rand(6), index=range(6), name='x')
>>> err_df = DataFrame(rand(6, 3) / 10, index=range(6), columns=['x', 'y', 'z'])
>>> s.plot(yerr=err_df)               # This works, errorbar appears using err_df x column

>>> s.index = tm.makeDateIndex(k=6)
>>> s.plot(yerr=err_df)               # But this doesn't, errorbar doesn't appear
```

can you rebase this on top of master again, then we can review
 Sure. Rebased.
 @TomAugspurger can you give a look see
 @sinhrks need a release note for this pls
 Rebased. I've thought release note is not requreid because error bar itself is new function of v0.14, and release note has following description. Should I prepare separate one?

> Added error bar support to the `.plot` method of `DataFrame` and `Series` (:issue:`3796`)
 oh just add this note number (or pr numbe) to that one 
 OK, added PR number.
 @sinhrks @TomAugspurger 
 Thanks @sinhrks. Could you rebase off master now that the Area plot is merged? There are a few conflicts in `tools/plotting.py` Other than the we're good.
  @TomAugspurger  Sure, rebased to latest master. Plotting test has been passed in Travis, and looks fail in offset test.
 pls rebase 1 more time, master is fixed up
 Thanks, rebased.
 Thanks.

@jreback a failure in `pandas.io.tests.test_packers.TestNumpy` and `pandas.io.tests.test_packers.TestNumpy` under the allowed failures build. Is this OK?
 @TomAugspurger yeh...thouse fail every once in a while on 3.4, not really sure why. if you would like to investigate give it a go!

otherwise this PR is ok
 Excellent. Thanks @sinhrks 
