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.
The affected commands are:  Write-Host, Measure-Command, Sort-Object, Where-Object, and Select-Object.

Note:  These changes were only made in functions that are called from either Mock or It.  If a caller has hidden the default versions of PowerShell commands elsewhere, it can still cause problems.