Fairly massive overhaul to support getting RPM inputs (extra meta-data,

prep/build/etc. scripts, doc files, dependency info) from a config file
rather than the dedicated "package_info" file.  (The idea is that
developers will provide RPM-specific info in the "[bdist_rpm]" section of
setup.cfg, but of course it could also be supplied in the other config
files, on the command line, or in the setup script -- or any mix of the
above.)

Major changes:
  * added a boatload of options to 'user_options' and
    'initialize_options()': 'distribution_name', 'group', 'release', ...
  * added 'finalize_package_data()', which takes the place of
    '_get_package_data()' -- except it's called from 'finalize_options()',
    not 'run()', so we have everything figured out before we actually run
    the command
  * added 'ensure_string()', 'ensure_string_list()', 'ensure_filename()';
    these take the place of '_check_string()' and friends.  (These actually
    look like really useful type-checking methods that could come in handy
    all over the Distutils; should consider moving them up to Command and
    using them in other command classes' 'finalize_options()' method for
    error-checking).
  * various cleanup, commentary, and adaptation to the new way of
    storing RPM info in '_make_spec_file()'