Consistent handing of axis label and tick fontsize

[philippjfr]

Also adds support for setting xticks and yticks size independently the same way xlabel and ylabel can be set independently. The bokeh backend wasn't respecting the axis label and tick font sizes previously.

[jlstevens]

Looks good - seems like a sensible extension. 'xticks' and 'yticks' are to 'ticks' what xlabel and ylabel are to labels. The pluralization is different but in this case it makes sense (xtick sounds like it is for a single tick label which would be wrong).

Merging.