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).
`in bytes` won't go in Javadoc, and it leaves (at least to me) the question of whether the block size is also in bytes, though I would assume it is.

How about documenting the units in the Javadoc for the variable, and maybe in the constructor argument description too?