Support for powershell scripts with whitespaces in path

This pull request fixes #177. Whitespace in path could be used if it goes right after the \` symbol. See [Hey, Scripting Guy! Blog post](http://blogs.technet.com/b/heyscriptingguy/archive/2012/08/07/powertip-run-a-powershell-script-with-space-in-the-path.aspx). Verified on PS1 and PS2.