htools: add a small module for crypto functions

This adds a dependency on the following new libraries:

- Data.HMAC module from the 'crypto' library
- Data.Text from the 'text' library
- Data.ByteString from the 'bytestring' library

In Debian, the bytestring library comes with the ghc compiler, whereas
the other two are available separately in wheezy/unstable.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>