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. 
this looks like a function that should be in `DOMUtils` (aka `Plottable.Utils.DOM`)