Added exclude_overlapping option in set_ticks to prevent ticks from overlapping

The default behavior is now that of matplotlib where tick labels are displayed whether or not they overlap. Addresses issue #84
Ah, I see you used ``try...except`` in the actual code, but it would be easier to just do it once here and then you can always import it from this module (so you only have to do ``try...except`` once instead of every time you need it.