Add base64, concat_binary, and as-is-binary key generators

New key generators:

* {base64, BinaryGenerator} will Base64-encode the binary created
  by BinaryGenerator.
* {concat_binary, GenOne, GenTwo} will concatenate the binaries
  created by the GenOne and GenTwo generators.
* Any literal binary B will create a fixed binary generator that
  always generates B.
Add example config that uses concat_binary & base64 key generators
Merge pull request #50 from basho/slf-string-key-generator-additions

Slf string key generator additions