ENH: Add mode method to Series and DataFrame
Closes #5367 - generally as fast or faster than value_counts (which makes sense because it has to construct Series), so should be relatively good performance-wise. Also, doesn't get stuck on value_counts' pathological case (huge array with # uniques close to/at the size of the array).

Not using result of hashtable.value_count() under the hood and instead iterating over klib table directly gives a _huge_ speedup. I just moved the value_count table creation method to a separate function (zero perf hit). For performance breakouts check out this gist:

https://gist.github.com/jtratner/7225878

DataFrame version delegates to Series' version at each level.

@jreback should I put this on hold until after 0.13?
 @jtratner seems reasonably tested.....i think its ok, need a doc mention? (in api, but near value_counts somewhere?)
 Yeah, I'll add it.
 Can you also add entries to api.rst?
 I missed putting it there. I'll do that--thanks for noticing that!
 added doc mention for Series and DataFrame
