allow unevaluated callables as bind values

[Mike Bayer]

change the util.callable(bind.value) checks to look at a separate field - i.e. if bind.callable_: etc. This so that callables, such as classes, can be passed as values to bind params which are handled by bind_processor().

I highly doubt anyone is using this behavior explicitly but its too critical, so its for 0.7.

[Mike Bayer]

ea55a4644c63358324ce4066c501c048c7a661a6

[Mike Bayer]

Removing milestone: 0.7.0 (automated comment)