Initial support for AWS spot instance requests.

This is a working patch to add AWS spot support to the 4.4 release of scalr.
There are a few things left to do, but before I do them, I wanted to make sure that my implementation was agreeable.
Things to do:
    Add more comments to the classes.
    Handle the case of an instance being terminated before a spot is fulfilled.  Given that it doesn't make sense to me that we support persistent spots (scalr will make a new spot request), it makes sense that all instances which were spots also have their spot request cancelled when the instance is cancelled, thus the original spot request id is preserved.