Switching prontserve add_job file uploads from string concatination to array appending for a 4x speed up (well, a 4x improvement according to pycallgraph on a 1.75Ghz Core i5. Possibly more on a rasperry pi.)

Removed all the unused slots and fallback to Cython.

Throwing an error when prontserve recieves a print command but there are no print jobs in the queue.