Add automatic variable highlighting to grammar.

This pull request includes:

* Highlighting of Powershell bitwise operators.
* More comparison operators like `-match` and `like`.
* Removal of Python language variables.
* Addition of automatic variables in Powershell which are highlighted as `variable.language` rather than `variable.other`. More information about automatic variables can be found [here](http://technet.microsoft.com/en-us/library/hh847768.aspx).
* Refactoring of tests for automatic variables.

I have unintentionally included the two previous "prepare release" commits by rebasing from `origin` instead of `upstream` but I think that these should be merged automatically (I hope).