Began moving UI functionality into dedicated Python module

This makes it possible to run Houdini centric tools in Maya and
vice-versa. The Houdini tools are almost all ported over. However,
the Maya tools are not yet abstracted (i.e. all the Maya UI stuff
is hard coded into the Maya utilities themselves).

This is the start of a general refactoring of code to clean up the
pipeline tools in general.
Merge pull request #88 from eestrada/master

Began abstracting UI code into separate module. I have tested these changes pretty thoroughly, and everything in Maya and Houdini seems to work just fine after moving around files and re-factoring code and whatnot. I would love some reviews on the re-factoring if anyone is willing to give them; I really want this (and future abstractions) to be air-tight.