Adding Hstore support package

Includes two Hstore types, for the common non-null values use case
(Hstore) and when nulls are necessary (HstoreWithNulls).

A map alias is not used to keep things obvious and allow for NULL
columns without adding more types and simplify Scan() parameters.

A test suite with a variety of values is also included.

(This may be a cleaner alternative to #136, which I saw while putting together this pull request)