Improve mysql_aes log filtering

This addresses two issues I ran in to using the latest crypt_keeper in my rails 4 project:
* Fixes  "invalid utf-8 byte sequence" exceptions trying to gsub on the binary encrypted data present in decrypt queries.
* Adds filtering for queries user-constructed queries that use mysql 5.6's from_base64 to do the full decryption in the original query. I did queries like this when returning large sets of data to avoid doing hundreds of separate decrypt queries. I wrote a helper to help generate queries like this which I could also contribute if desired.

I haven't looked into it yet, but I'd love to be able to do mysql_aes decryption at the app layer without using mysql at all. I'd assume this is possible since mysql's aes algorithm seems more simplistic than openssl's, but I haven't looked into it yet.