Build script improvements:

Partially addresses #103

* Remove the build batch files as all functionality is in the script
  It will also be assumed that the person using the script has Python
  properly set up, in which case no batch files are needed.
* Added extra help from the batch files into the script command line
  parser
* Added required argument --release, to specify the Wrye Bash release,
  as it is no longer feasible to get it from bash.py