Fix for Issue #42 (non-numeric "delphi" version in build options)

Fixes the issue in Delphi.build() method of specifying delphi version using human meaningful version where that is non-numeric.  Introduced constants to represent "internal" numeric versions and methods to translate supported delphi version flag to the appropriate constant and to convert those constants into meaningful version names.

Apologies if this is a bit scrappy, particularly the commit message.  This is my first time putting together a contribution to a project using Git/GitHub.  Any advice for going about it differently in the future will be gratefully received.