GHC 7.4 support

I tried compiling on GHC 7.4.2, but the cabal file had restrictions on bytestring and base. Once I relaxed the constraints, the code compiled and ran just fine. (It also seems like resourcet was not a needed dependency, so I removed it.)

As a separate point, I was curious how sandi would compare performance-wise to base64-bytestring and base64-conduit. I put together a [small benchmark](https://gist.github.com/snoyberg/5075429). I didn't spend much time on it, so there may be flaws, but it seems to indicate that all three libraries are in the same ballpark of performance, with base64-bytestring being faster on smaller datasets and sandi faster on larger datasets.

Nice package!