Refactoring of VaspToDbTaskDrone

Backwards incompatible (just for subclasses) - I'd like someone from MP to sign off on this first (no nosetests in MPWorks). 

Allow for more flexible custodianformats (different numbers of runs, etc).
BACKWARDS INCOMPATIBLE!! All classmethods have been removed (and
therefore inheriting classes needs to be adjusted accordingly)
Other changes *should* be backwards compatible, except for edge cases
where extra vaspruns (in addition to relax1 and relax2) are found in
the run folder.