Fix Team City builds

the & made the nUnit.xml impossible to parse. quick-fixing this. There
is #122 issue for this.
Fix internal module support to work on PowerShell 2

Also fixing the TC build again
Fix Parse-ShouldArgs in PowerShell strict mode

Parse-ShouldArgs now works properly if StrictMode has been enabled, performing bounds checking on the $shouldArgs array before indexing.  It also throws an error if no AssertionMethod was passed (ie, ( | Should ) or ( | Should not )).
Fixes #76
Validation of Assertion Method in Should

Loosely related to the fix in the Issue76 branch, this update validates user input to the Should function.  Instead of producing the stock command not found exception, Should will now produce an error of "'BeNulOrEmpty' is not a valid Should operator." (where the user had misspelled BeNullOrEmpty, in this case.)