Remove some grammar elements that don't apply to Powershell.

This pull request includes the following changes:

* Remove a number of elements that apply to Python (or other) syntax but not to Powershell
 * `import`, `global`, `pass`, `yield`, `default` and `with` have been removed
 * `do` and `until` have been added

* Add hyphen for logical operators (Powershell does not use them without a hyphen). Add `xor` operator. These are explained in more detail in [Powershell logical operators](http://technet.microsoft.com/en-us/library/hh847789.aspx)

* Add tests for 'do-until' and 'if-else' keywords.