Updates to plugin handling

- Removed WBrowser, Debug and Log plugins from being loaded by default.
  These are now loaded from the "Help" menu.
- Unified startup/shutdown of local and global plugins via PluginManager
- Added Close buttons to WBrowser, Debug and Log plugins
- updated documentation
- Added FITSpkg and WCSpkg configuration options in general.cfg
  - these control which package you want to use for WCS and loading FITS files
- If sphinx is installed and you do a "make install" in the doc directory
    it will copy the documentation into the ginga/doc directory where it
    becomes part of the package install:
      cd doc; make install
      cd ..; python setup.py install