Added support for mocking:
* internal module methods
* alias commands
* application commands
* filter commands

Added partial mocking support for:
* fully-qualified command names (fully qualified cmdlet invocations are not supported)
* external script commands
  The only supported scenarios are when the script is invoked using one of the following methods:
    scriptName.ps1
    & scriptName.ps1
    . scriptName.ps1

Added corresponding unit tests.