Several fixes and improvements

* Add Topic model
* Add PDB post-mortem debugging
* Add logger to plugins (`self.log`)
* Add NLTK resource auto-download
* Force installation of requirements even if adding doesn't work
* Add a method to find files in several possible locations. Now the plugin.open
method will try these locations IF the file is to be opened in read mode.
Otherwise only the SENPY_DATA folder will be used (to avoid writing to the
package folder).

senpy/__main__.py

@@ -130,7 +130,7 @@ def main():
         return
     sp.activate_all()
     if args.only_test:
-        easy_test(sp.plugins())
+        easy_test(sp.plugins(), debug=args.debug)
         return
     print('Senpy version {}'.format(senpy.__version__))
     print('Server running on port %s:%d. Ctrl+C to quit' % (args.host,