Adds underscore motion command

Acts like ^ (first non blank in a line) but with numeric arguments greater than 1 it will go down (numeric argument - 1) lines and position the cursor on the first non blank of that line. Otherwise it just goes to the first non blank in a line.
Merge pull request #322 from weaksauce/underscoreMotion

Adds underscore motion command
Merge commit '03b2c' into refactoring

* commit '03b2c':
  Adds underscore motion command

Conflicts:
	XVim/XVimMotionEvaluator.m