Minor ticks

This is an initial implementation of minor ticks. It appears to work so far for scalar and angle coordinates though I haven't added tests or documentation for it yet. Functionality displayed here:
http://nbviewer.ipython.org/gist/anizami/91d5252b8f0182734209
Ah I didn't see that simplification. No it works, ```AngleFormatterLocator``` already has ```self._unit``` equal to ```u.degree```