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
@astrofrog - I'm having a little trouble adding unit tests for ScalarFormatterLocator. Since the ```values``` and ```spacing``` arguments for ```minor_locator``` are taken from  the ```locator``` method. If I do something like
```python
values, spacing = fl.locator(34.3, 55.4)
minor_values = fl.minor_locator(values, spacing, 5, 34.3, 55.4)
```
```minor_values``` does not [36., 37., 38., 39., 41., 42., 43., 44., 46., 47., 48., 49., 51., 52., 53., 54.] (which should be the expected behavior) but also has the values 40. and 45 (which shouldn't be there because they're major tick values) and I've narrowed this down to a rounding off issue - ```spacing.value=5.000000000000001```. Is there a good way to round off values like these?