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. 
are you sure the classing is handled properly here? i thought our convention is to have the `TICK_LABEL_CLASS` applied to the `g` containing the tick text and tick line, and then have the `text` without an explicit class.
also, please consider using the TextUtils funcitonality, eg. `getTextHeight` which is already quasi-pure when called on the parent `g`, so you don't need to deal with removing the test text element by hand.