This test simply loaded an old example of pickled data.  It was both extremely
indirect and brittle.  Changes to the code base will eventually invalidate
nearly all of the old classes used, and whatever the exact issue was, it
should've been tested directly.

The temporary values created during subtensor broadcast-value computations would
cause the test value computation steps to complain/err.  This change prevents
that.

This change also fixes the `set_subtensor` boolean gradient bug in #105.

This change prevents downstream code (e.g. test value computation) from throwing
exceptions (e.g. `ValueError`s) that break the logic in
`theano.tensor.basic.as_tensor_variable`.

- Fix `*Subtensor*` `Op` shape inference
- Make `theano.tensor.basic.as_tensor_variable` consistent in how it handles constant conversions
- Remove `TensorConstant` caching

Now, when `as_tensor_variable` is given `Constant` types, it will return the
same result as a call with the underlying non-`Variable` data.

Closes #98

Closes #99

The merge-based functions were moved to `toolbox` to reduce unnecessary
cross-module dependencies (especially among core modules).  `equal_computations`
was moved from `scan_module` because it provides a basic graph object `__eq__`
implementation and it has multiple references outside of its own
module/sub-package.

Combining checks into one and dealing with skipped runs