Fix comparison operator matching

This fixes 2 issues with powershell comparison operators matching in the grammar:

1. The operators were being highlighted if they were part of a word (from issue #26)
2. The operators currently only match if what's typed is lower-case, but they should be case-insensitive.

These commits seem to get the word boundary stuff working:

![image](https://cloud.githubusercontent.com/assets/238079/5188539/d5e8686c-74a3-11e4-829f-106ac469e1bb.png)

Before, `-equal` would have `-eq` highlighted but not the rest of the word.

[Comparison operators PowerShell reference](http://technet.microsoft.com/en-us/library/hh847759.aspx)