Moved UI elements to UI namespace

Closes Issue #44.

As there were changes to the namespaces, there may be issues with registration. I didn't seem to have any problems, but be prepared for it.

If you have any workbooks referencing Rubberduck.AssertClass, you will need to remove the reference, save and close, then open and re-add the reference. Otherwise an ActiveX error occurs and all tests return inconclusive.