Git payload expansion

Created three fields for json serialization, and initialized them in a new function, initFields, which is encapsulated by another function,  extractPayload, which returns a GithubPayload object.

NOTE: All payloads are assumed to be Github payloads. This is to be changed asap, such that the payload type will be validated so it can be determined what kind of payload it is (i.e. github or travis, the two payloads intended to be supported in the immediate future).