Add conversion instances for `ShortByteString`s
Add conversion instances for `ShortByteString`s

As `Data.Bytestring.Short` is only available for `bytestring >= 0.10.4`,
the `ToField`/`FromField` instances are conditionally defined as well
based on the `bytestring` version.

Addresses #91

hvr/cassava,treeowl/cassava,tibbe/cassava,richardfergie/cassava,psibi/cassava,bgamari/cassava,bergmark/cassava,jb55/cassava