Use memory alignment so that SSE2 can be used with dSFMT

[bashtage]

dSFMT fails using generic memory alignment since there are no guarantees that memory is aligned on a 16 byte boundary when allocated by python. This means that is will probably be necessary to directly allocate memory for the state so that it can be over provisioned to guarantee alignment.

This will require a non-trivial change and will probably necessitate adding __dealloc__ to the RandomState class as well as PRNG-specific state initializers/destructors, e.g. _alloc_state(rng_state* rng) and _dealloc_state(rng_state* rng). This path might also require adding a variable or two to contain the original pointer so that it can be freed.