Aligned allocation

This seems like a very nice and useful little library. Definitely fills a need.

Would you be willing to include support for aligned allocation? This is handled a little differently on each platform, and providing a nice, platform independent interface to it that "just works" would be quite useful.

For instance, if you see lines 12-43 in [this file](https://github.com/sampotter/python-embree/blob/master/embree.pyx), you can see my own attempt at this. Frankly, I have no confidence that I've done this the right way. :-)

(I'm considering using cymem in the library linked to above. Intel's Embree library requires memory to be allocated with alignment.)