Add query support to Hstore

[ilopmar]

Add query support to Hstore type

[Alotor]

Queries to be supported:

• Has Key The hstore in the table contains the specified key
• Has All Keys The hstore in the table contains all the keys passed as parameter.
• Has elements The hstore in the table contains all the elements in the query
• Is contained The hstore in the table is a subset of the specified as parameter

[ilopmar]

Do you have the postgres documentation link with the description of this queries?

[Alotor]

http://www.postgresql.org/docs/9.0/static/hstore.html

• exist(hstore,text) does hstore contain key?
• hstore @> hstore does left operand contain right?
• hstore <@ hstore is left operand contained in right?

[ilopmar]

Created tasks #16, #17 and #18