Fully-qualified commands

Fixes #73

Several calls to built-in PowerShell commands are now module-qualified, making Pester behave properly internally, even if the caller has mocked these commands.

Note:  These changes are only made in functions called by Context, Mock and It.  If a caller has hidden default PowerShell commands outside of a Describe block, they could still cause tests not to work properly.
Merge branch 'MockSafety' of https://github.com/dlwyatt/Pester into MockSafety