TEST: Allow runspace initial session state to be configured for TestHost.
Allow initial session state variables to be accessed from runspace.
Make cmdlets defined in initial session state available to runspace.
REFACTOR: Remove duplicate code when loading cmdlet information.
Merge pull request #147 from mrward/initial-session-state

Initial session state - variables and modules
Allow cmdlet to invoke a script.

Implement PSCmdlet.InvokeCommand.InvokeScript()
Merge pull request #148 from mrward/invoke-script

Allow cmdlet to invoke a script.