Fix SymEngine code generation. Closes #24.

* replaces mat / scalar with mat * scalar**-1.
* replace list argument to subs with a dict

(Although there are pending changes to SymEngine for the above two
issues, they aren't released in a version yet.)

* Removed deep as a kwarg to expand(). It doesn't do what we want
anyway.

* Disabled caching for tests in test_kernels. This exercises SymEngine
code generation every time the tests run. The execution time for these
tests without caching is about 6 minutes for me, so the overhead is
acceptable.