Introduce a slightly faster binary to hex conversion.

This commit introduces a slightly faster hex conversion function:

Before:

    2> kinetic_aws:bench(10000).
    3030014 us
    ok
    3> kinetic_aws:bench(10000).
    3113934 us
    ok
    4> kinetic_aws:bench(10000).
    3037627 us

After:

    1> kinetic_aws:bench(10000).
    2847586 us
    ok
    2> kinetic_aws:bench(10000).
    2854246 us
    ok
    3> kinetic_aws:bench(10000).
    2960231 us
    ok

This does change the signature, as reflected in the test modification.
I'm unsure if this is a problem.

Signed-off-by: Brian L. Troutwine <brian.troutwine@adroll.com>
The data that goes into the hash is the same though, so I'd expect the same output. I think the increment in performance is awesome and I'd want that :).