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
@anizami - there are ways to compare floating point values to within a tolerance, but I think that we probably don't want to do that here. A cleaner approach would be to figure out what the first major tick position will be (by finding ``imin_major`` the same way as it's done in the major locator) then to filter based on the index rather than the values themselves. Basically, if you know that the first major tick is the 4th value, then values[3::frequency] will give you the major tick positions, and so if you find a way to invert that slice you will get the minor ticks.

As an aside, it may be worth seeing whether rather than having two methods for the locators for minor and major ticks, you could simply combine them into one, since some of the logic will be repeated?