Add bindings to verify_32 and verify_16

These provide constant time comparison operations for short strings of
32 and 16 bytes, respectively.  The relevent functions in this library
already use such comparisons (actually at the C level), but they are
exposed for completeness.
verify_(32/16) => verify(32|16)

To be a little more 'ruby like'
Merge pull request #13 from namelessjon/dev/verify

Add bindings to verify_32 and verify_16