ENH: Implement "standard error of the mean"

As discussed in issue #6897, here is a pull request implemented "standard error of the mean" (sem) calculations in pandas.  This is an extremely common but very simple statistical test.  It is used pretty universally in scientific data analysis.

sem is implemented in nanops, generic, and groupby.  Unit tests are included and the docs have been updated.

I have also included an improved .gitignore, based on matplotlib's but including everything from the old .gitignore (hopefully better organized now, though).