Improvements to DscConfiguration module

Credentials code has been cleaned up a bit; options other than a module load argument have been added to manage the certificate used for encryption / decryption, and various error handling / reporting has been revised.

Also made the same changes that were done in the DscBuild module, removing calls to Invoke-Expression when importing psd1 files.

Bumped version of both DscConfiguration and DscBuild (which I forgot to do in the earlier update.)