Fix uncompressed option and clean up build script

- -c (combine only) option of snowpak.sh is no longer broken.
  Previously the validate_options function only contained the paths to
  correctly validate the -y (YUICompressor) option.
- Extraneous exit statements from parse_args have been removed; these
  statemetns were unreachable as calls to usage already result in an
  exit.
- If no options have been provided an error message is now printed along
  with usage.  Previously this resulted in a failure during validation
  and would print an unrelated error message.