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
I wonder whether we could avoid repeated code between the two ``minor_locator`` implementations by simply setting ``self._unit`` to ``u.degree`` and using the implementation for the scalar case in the base class for the formatter/locators? Or would this not work?