Add support for `Data.ByteString.Short`

The short bytestring has been added with bytestring-0.10.4 (as shipped with
GHC 7.8.x). Due to internal changes in the ByteArray# representation, the
ShortByteString value constructor has an additional field with base<4.3
(i.e. GHC <7.0), this has been taken care of by CPP checks.

A simple property-check has been added to the testsuite in order to exercise
some of the new code-paths.

NB: This requires at least a minor version bump

This addresses tibbe/hashable#79