API: add subplot_kws to FacetGrid

Fixes #311.
Add hue_kws to FacetGrid and markers to lmplot

This lets the user map the hue variable with arbitrary additional plot
aesthetics in FacetGrid, and it provides an easy interface to map the
scatterplot marker when using lmplot.

This also has a small enhancement to change the marker params depending on the
kind of marker so that line-based glyphs are not invisible (which would
otherwise be the case with the default seaborn rc Params)
Document hue_kws and markers in docstrings
Add hue_kws and markers to PairGrid and pairplot
Document markers and hue_kws
Merge pull request #323 from mwaskom/lmplot_markerstyle

Add hue_kws and markers to axis grids
Merge branch 'heatmap' of github.com:olgabot/seaborn into olgabot-heatmap
Rename source file with heatmap
Add utility functions to check for ticklabel overlaps
Refactor heatmap function and add features
Fix ticklabels test on Python 3
Ensure that the pcolormesh covers the full axis limits
Plot the matrix with rows running from top to bottom
Add tests for heatmap
Use matplotlibrc file to force Agg backend in tests

This will avoid errors in tests that need a rendered plot. It might not
be the best way to force the Agg backend, but seems like the obvious way
to do it. I'd rather not keep the file in the root directory of the package
because lots of times I plot from within that directory and it would lead
to confusing issues.
Be more thorough in the annotation test
Add gallery example for heatmap
Add ability to set equal aspect in heatmap call
Pass read_csv kwargs in load_dataset
Add correlation matrix heatmap example