EncryptedRandomAccessThing and other API's from my GSoC project

This is EncryptedRandomAccessThing which toad will be using in his db4o replacement along with around half of the API I have written this summer for GSoC. ERAT is dependent on the three API's included. I am still in the process of writing docs and unit tests for ERAT but the other three APIs are complete. These will be done this weekend. This requires BouncyCastle 1.51 (#271).
I'm not sure if it's really needed, but I put it in as a way of saying, if you're writing something new using this API, it's suggested you use this type. Kind of as a way to try and uniformly use the same algs across Freenet as possible.