gplugin/gplugin

Add an id property to GPluginLoader and add tests to GPluginLoader

Testing Done:
Ran the unit tests

Reviewed at https://reviews.imfreedom.org/r/895/
REVIEWBOARD_URL = "https://reviews.imfreedom.org/"
REPOSITORY = "gplugin/gplugin"
REPOSITORY_TYPE = "mercurial"
BRANCH = "default"
LAND_DEST_BRANCH = "default"
LAND_PUSH = False