Adding Facebook module to project
Setup AndroidManifest after addition of Facebook SDK
Set up basic UI for Facebook login

Main activity uses two fragments in layout. One fragment
for unauthorised session state, and another for authorised
session state.
Correcting Facebook module import in project
Setting up Facebook authentication logic

Can now successfully log in via Facebook credentials.
This is only a basic implementation that follows the
tutorial steps. Further work will be take place.

Currently no way to log out - may need to revise this.
Merge branch 'facebook/login' into develop