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).
@toad and I discussed this and I think a good solution is that `isClosed` have an associated reader-writer lock, and `close()` grab a writer lock and the others grab a reader lock.