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).

tests/test_extensions.py

@@ -47,7 +47,7 @@ class ExtensionsTest(TestCase):
 
     def test_add_delete(self):
         '''Should be able to add and delete new plugins. '''
-        new = plugins.Plugin(name='new', description='new', version=0)
+        new = plugins.Analysis(name='new', description='new', version=0)
         self.senpy.add_plugin(new)
         assert new in self.senpy.plugins()
         self.senpy.delete_plugin(new)