switch from inheritance to directly overriding methods for caffe.Net

Also, internally drop the "Caffe" prefix for classes exported by the
internal _caffe module. This change means that SGDSolver will
automatically return a Net with the "nice" Python interface rather than
the internal one, (and avoids the extra layer of method wrapping that
results from switching to composition).
pycaffe: expose SGDSolver.net
pycaffe: introduce CheckFile helper
pycaffe: expose SGDSolver.solve