[FEATURE] add support for key-value store in user object

Using postgres HSTORE datatype. The db-migration will enable the HSTORE extension to your database, but the user that is used needs to have the SUPERUSER role.

You can alter a user's role with following command in psql
psql> ALTER ROLE postgres SUPERUSER;
Merge branch 'data_hstore' of github.com:geertweening/ripple-gateway into data_hstore
[CHORE] Bump version after updating user table migration with psql extension.