Add specific config stuff of plugin being set in the settings file.

Some minor changes for the ParameterPlugin interface to make it
receive a config parameter. This config is get from the settings
file and is specific of each plug in(can have anything).
Also make the openrtb plugin independent of the mopub exchange. The
next step will be to make rubicon from this one.
Aslo add initial Readme file.
Add mopub specific settings for the openrtb plugin under
settings/mopub_config.py
Also click notifications for the openrtb plugin.