Add server plugin system (#1447)

* Add server plugin system

Plugins are optional modules loaded at runtime. Specification:
* Plugins are loaded from the server.plugins module (directory
  server/plugins)
* The import_plugins method is run as part of the initialization of the
  server module in __init__.py

* Add plugins to the EB build process

* Remove bit of dead code

* Respond to feedback from @bmccandless

server/test/test_cxg_adaptor.py

@@ -7,7 +7,6 @@ from server.test.test_datasets.fixtures import pbmc3k_colors
 
 
 class TestCxgAdaptor(unittest.TestCase):
-
     def test_get_colors(self):
         data = self.get_data("pbmc3k.cxg")
         self.assertDictEqual(data.get_colors(), pbmc3k_colors)