* Add taint args to buffer creation
Allows the `new_symbolic_buffer` and `symbolicate_buffer` functions to take keyword args for tainting. Defaults to frozenset in both cases.
* Add unit tests and ArrayProxy taint propery
Adds simple unit tests for tainted buffers. Added a property to the ArrayProxy class in smtlib.expression so that it's possible to access the taint of the proxied ArrayVariable.
* Updated docstrings