Corrected a picking out issue.
Added initial setup of plugin macros.
Defined 'wintermute_plugin_declare' in CMake.

   * Provided the necessary CMake command to build a plug-in for Wintermute.
Corrected Git version info collection.
Proved `make uninstall`.

  * Allowed projects using Wintermute to automatically get an uninstallation target.
Updated Git refspec in source code.

  * Updated WintermuteGlobals with correct refspec constant.
  * Updated Wintermute's application version with correct refspec constant.
Dropped Git dependency for Wintermute.
Added three methods for WintermutePluginMethods.

  * Added implementation for methods:
   - `wintermute_plugin_declare`
   - `wintermute_plugin_configure`
   - `wintermute_plugin_set_version`
   - `wintermute_plugin_install`