Adding np.nanmean(), np.nanstd(), np.nanvar()
Added tests for nanmean(), nanvar(), nanstd()
Tests now checks the warning state
Updated comments and dtype tests in _methods.py
MAINT: Separate nan functions into their own module.

New files lib/nanfunctions.py and lib/tests/test_nanfunctions.py are
added and both the previous and new nan functions and tests are moved
into them.

The existing nan functions moved from lib/function_base are:

    nansum, nanmin, nanmax, nanargmin, nanargmax

The added nan functions moved from core/numeric are:

    nanmean, nanvar, nanstd
MAINT: Clean up core/_methods.py and core/fromnumeric.py

Use issubclass instead of issubdtype.
Add some blank lines.
Remove trailing whitespace.
Remove uneeded float casts since true_divide is default.
Clean up documentation a bit.