NetSpec: type-check Function inputs (they must be Top instances)

[jeffdonahue]

This adds simple type-checking for a common (at least for me) class of NetSpec use errors: the use of non-Top instances as inputs to Functions. The exception is raised when the Function is __init__ed rather than lazily when to_proto is called, making it easier to debug (IMO).

[jeffdonahue]

(Fails Python3 Travis checks due to apparently different behavior of either regexps or unittest.assertRaisesRegexp; will look into debugging that pending @longjon's general approval of this. fixed, and I was of course wrong about the cause -- in Python 3 print ('%s' % type(())) prints <class 'tuple'> rather than <type 'tuple'>, so the regex needed to be generalized to handle that.)

[shelhamer]

Thanks for the net spec check Jeff!