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/eb/Makefile

@@ -41,6 +41,9 @@ build: clean
 	      cp -r customize/ebextensions/* artifact.dir/.ebextensions;  \
 	  fi; \
 	fi; \
+	if [ -d customize/plugins ] ; then \
+	  cp -r customize/plugins artifact.dir/server/;  \
+	fi; \
 	(cd artifact.dir; \
 	  cp -r server/common/web/static static; \
 	  zip -r ../artifact.zip . --exclude server/test/\* server/eb/\* ; ); \