Improved Prepare-Payload (now Out-EncodedCommand)

* Renamed Prepare-Payload to Out-EncodedCommand in order to conform to a
standard cmdlet verb.
* Fixed bug in PowerShell v2
* Defaults to full base-64 encoding unless it exceeds the cmd.exe
character limit. Otherwise, it will default to partial base-64 encoding
in an effort to save space. Thanks @Carlos_Perez for the idea!
* User will be prompted if the cmd.exe character limit is exceeded.
* Command-line output uses truncated arguments in order to save space.
Thanks @obscuresec!