KeyGenUtils API

This is the API I've written for generating Keys, nonces and ivs. It will also convert byte arrays to into SecretKeys, KeyPairs, and a couple other classes. 
Any reason for not using the plain old `IllegalArgumentException`? I think the types in `KeyPairType` (except for DSA) should always be present on a Freenet installation, so forcing the user to wrap his code with `try(…) catch(…)` would unnecessarily increase the number of lines in the consuming code.