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 were only made in functions that are called from either Context, Mock or It.  If a caller has hidden the default versions of PowerShell commands elsewhere, it can still cause problems.  It's possible to qualify all such commands in the Pester module, if that's desirable, but that would be a much higher number of changes.