BLD: add python 3.4 build on travis

BLD: remove statsmodels from 3.4 build

BLD: allow pip/setuptools to work on latest versions

BLD: install nose from master on EXPERIMENTAL builds
Merge pull request #7009 from jreback/python3.4

BLD/TST: Python3.4 testing via travis
Added selection example to cookbook

I added a (for me) very helpful Q&A from Stackoverflow on groupby/minimization while retaining the original DataFrame columns. Additionally, I cleaned up some links in the same cookbok section.

(Disclaimer: this is my first pull request.)
Merge pull request #7012 from ojdo/patch-1

Added selection example to cookbook
DOC: cookbook.rst corrections
ENH: cythonize groupby count
CLN: remove ipdb
CLN: spacing in generated .py
BUG: wrap _count to convert back to int64