adding optional use of cloudpickle module

[fbnrst]

A PR from a clean branch based on the discussion in #35.

[ihrke]

Thanks, again!

[ihrke]

btw, just thought about something: what happens if you pickle.load() a lambda-function that has been pickled with cloudpickle? This is what always happens, the way we are currently handling it...

[fbnrst]

This works:
https://github.com/cloudpipe/cloudpickle#examples

[rossant]

thanks @fabianrost84 !

[fbnrst]

Now, I was wondering if the except clause should only handle ImportError. Could there possibly be another exception for that we want to fall back to the standard pickle module for dumping? E.g. a broken cloudpickle module that can be imported but does not provide a dumpfunction. So should we maybe let all exceptions being handled like that:

try:
   import cloudpickle
   dump = cloudpickle.dump
except:
   dump = pickle.dump