Add toggle `CategoryAxis.textOrientation` 

Allows you to set text orientation.

Added quicktest.

Close #504 although I think we should do some testing of whether the vertical alignment option works very well (tkim could you play around with it and see if the behavior is up to par?)

Conflicts:
	plottable-dev.d.ts
	plottable.d.ts
	plottable.js
	quicktests/list_of_quicktests.json
Why are auto and horizontal the same?
Shouldn't the code flow be something like...
if this._textOrientation is auto
  set orientation to horizontal if axis is horizontal else set orientation to vertical
?