Time Axis and Formatter

Reopening #562 now that #564 has been merged in. 

I've removed the Time Scale changes for now because they weren't playing nice with the domainers #553  (since the domainer assumes that they are quantitive scale). 

For the Time formatter, I wasn't too sure what to do with precision. Currently I'm ignoring it, but we may want to make it do something in the future. 
The reverse dot naming scheme has caused lots of trouble as far as documentation is concerned -- it'd be a lot less confusing, but more wordy, to say `Plottable.Axis.TimeAxis`, rather than `Plottable.Axis.Time`. However, I'd rather avoid another Great Renaming. Compromise proposal:

To avoid confusion, in documentation we reverse the dot-naming and replace the dots with spaces. Examples:
- `Plottable.Axis.Time` --> "Time Axis"
- `Plottable.Scale.Linear` --> "Linear Scale"

etc.