GH-260 Add variable that stores what OSes are supported by each plugin

- All plugins now have a plugin.os variable which is a list of OSes
  as defined by the python sys.platform API:
    http://docs.python.org/2/library/sys.html#sys.platform
- Remove functions that aren't used by Freeseer

Freeseer/freeseer#260
GH-260 Make plugins only load if plugin is supported on user's OS

- plugin.py made functions that return a list of plugins for the various
  categories that are supported by the user's OS
- Updated Freeseer GUI to use these new functions

Freeseer/freeseer#260
Closes GH-269 Plugin manager should provide api for reading and writing config

- Added new APIs:
    get_plugin_option - provides api to retrieve stored configuration option
                        for plugin
    set_plugin_option - provides api to set a configuration option for plugin

Freeseer/freeseer#269
GH-260 List both linux and linux2 as supported os in linux plugins

- Per discussion in GH-268 list linux and linux2 as supported OSes
  for plugins that support Linux. This allows the support of both
  python2 and python3 formats.
- Revert list() back to [] for OS list declaration.
- Remove unused os import in plugin

Freeseer/freeseer#260
Closes GH-260 Make plugins only load if plugin supports user's OS

Closes #260
Closes #268
Closes #269