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