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
At the top, add a comment to say "This is a backport of the ``rc_context`` class from matplotlib 1.3. Also, I think we should import the real one if available, so:

```
try:
    from matplotlib import rc_context
except ImportError:
    class rc_context(object):
        """
        ...
```

Does this make sense?