ENH: cythonize groupby.count
closes #7003
- [x] vbench
- ~~axis parameter~~ (this only works in non-cython land)
- [x] tests for `object` dtype if there are none
- [x] datetime64/timedelta64 count

vbench results:

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_multi_count                          |   7.3980 | 6814.2579 |   0.0011 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
```

@jreback axis parameter has no effect here...i kept for back compat with `test_multilevel`... i think should prob be implemented (i'm happy to do it)
 Yeh not sure what it does in a groupby

as an aside why do u still need the lambda expression in count? is it called?
 oh actually u need to pass a `npfunc` argument to `_groupby_function` ... fallback
 fallback if the dtype does work / match?

FYI I don't think group count will work for datetime64; need to convert to i8 view and then compare against tslib.iNaT for nulls 
 fallback is called if any exceptions occur in the cython call
 usually that is a dtype mismatch 
 @jreback supporting date-likes here i think requires more than i originally set out to do with this ... about to push object dtype for counts
 I think datelikes DO need to be handled (you can't just take a i8 view when you pass in?) and then do your comparisons instead of 

`if val != val`

`if val != tslib.iNaT`

for nan testing?
 Yeah it's there. I was trying to implement ttimedelta arithmetic for groupby but the py26 oddities are in the way. I'll do it in another pr
 Sorry the numpy 1.6 oddities are there
 ok...no problem (or leave it in python land). is ok too for now; can't leave it hanging though because would break things, ahh yes, the 2.6 oddities, so maybe you can just make it use python and M8/m8 in 2.6 (e.g. have a branch in the code), ugly but what the hey
 @jreback yep took out the cython attempt ... now just squashing ... making sure that the cython path is hit for dates/timedeltas (but only `count`); current behavior is to raise ... i think  there's an issue somewhere about groupby with deltas
 vbench?
 vbench coming soon
 @jreback 

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_multi_count_dates                    |  20.4120 | 15484.0871 |   0.0013 |
groupby_multi_count                          |  20.8774 | 15609.8307 |   0.0013 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
```

758x speedup, think we're doing ok :)
 ah object dtype is still slow , might be another one of my dumb typos
 can you make the vbench a bit smaller (so the base case takes say 1/10 the time)? (shouldn't really affect the speedup much), which is nice!
 sure thing
 when you do v0.14.0, put in the perf section FYI (release notes goes in improvements)
 will do thanks for the heads up
 @jreback can we get this in soon as green? want to avoid doc merge conflicts.
 sure go ahead on green
 thinking about this

int types can never be nan at all
and only int64 need comparisons against iNaT, so if less than int64 you don't even need a routine you can just call size (eg it's just the size of the group themselves -/ can't have Nan's)

object types need comparison against the actual NaT value (I think) 

wonder if any of that even matters
 Right, really only float, object, and int64 and those are really a view on dates or time deltas which are just ints really. Could have size as the fallback function instead of current version which computes sum of nonnull. Then if I take out the generated for ints it will raise and use the fallback. 
 yep
sorry should have mentioned before
 I can do some microbenchmarks. If size is done with Len  then we go from linear in the number of rows to constant so def worth an investigation
 Pandas isn't on iPhone so this may have to wait until tomorrow :/
 hah

no size is already computed (when the groupby happens) so should be trivial

more worried about the issue of the type coercion in the current cython code vs iNaT which is a bigger value than say int8 
though it prob does work
 @jreback i have the pr with `size` do u want me to put it up?
 sure
