Backend cleanup part 1: Reduce size of interface.

Some methods didn't belong, others were unused and may return.
Backend cleanup part 2: Sort and document all methods.

Several minor changes for better consistency.
Backend cleanup part 3: Rename 'KegbotApi' interface to 'Backend'.

Adds 'BackendException' hierarchy.
Backend cleanup part 4: Refactor two backend methods.
Add http-request library.

Part of issue #50.
Update checkin client.
Update bug reporter.

E-mail is no longer required, as several users did not
have an installed app.
Add OkConnectionFactory.
Fix authentication dialog theme.
registration: Streamline registration.
CameraFragment: Behave correctly in absence of auto focus.