fix 'compress' semantics

The type and semantics of the 'compress' keyword argument were ambiguous. In 
the joblib/memory.py module the type was listed as boolean in the docstring
and as default value for the keyword arguments. However, in
joblib/numpy_pickle.py it turned into an integer which eventually becomes a
compression level for zlib. This never showed up anywhere I guess, since by
virtue of duck typing False was interpreted correctly to mean a compression
level of 0 and True to mean 1.

Make the 'compress' setting uniform across joblib/memory.py and 
joblib/numpy_pickle.py. Adapt the docstrings and the keyword defaults to be 
integers all the way.