Inherit default grid style from matplotlib.rcParams

Based on `matplotlib.axis.YTick._get_gridline`. Note that Matplotlib
implements gridlines with `Line2D`, whereas wcsaxes implements gridlines
with `PathPatch`. The two have slightly different option styles
regarding dashes, so we have to map between them.
Inherit default tick style from `matplotlib.rcParams`

Note that Matplotlib's defaults dictionary contains default settings
only for `xtick.*` and `ytick.*`. Because there is not a fixed mapping
between WCS axes and Matplotlib `x` and `y` axes, the best we can do is
arbitrarily pick one axis from which to inherit defaults.

This is explained in the docstring.
Add unit test for default grid and tick style
Add argument to set tick width

And update original unit tests to explicitly set tick width and size,
overriding default values, so that they continue to pass.
Merge pull request #49 from lpsinger/rcparams

Inherit default grid and tick style from `matplotlib.rcParams`