Add support for database extensions.

This commit adds support for database extensions via
salt.states.postgres_extension

When configuring database pillar data all you need to do is add (optional)
extension list with the extensions that you want the state to apply to specific
database. Example:

    db1:
      owner: 'localUser'
      user: 'localUser'
      template: 'template0'
      lc_ctype: 'C.UTF-8'
      lc_collate: 'C.UTF-8'
      extensions:
        - uuid-ossp

This will make sure `uuid-ossp` extension is enabled on `db1` database.

Updated pillar.example to include (optional) extensions

pillar.example

@@ -38,14 +38,18 @@ postgres:
       template: 'template0'
       lc_ctype: 'C.UTF-8'
       lc_collate: 'C.UTF-8'
-
+      # optional extensions to enable on database
+      extensions:
+        - uuid-ossp
     db2:
       owner: 'localUser'
       user: 'remoteUser'
       template: 'template0'
       lc_ctype: 'C.UTF-8'
       lc_collate: 'C.UTF-8'
-
+      # optional extensions to enable on database
+      extensions:
+        - postgis
   # This section will append your configuration to postgresql.conf.
   postgresconf: |
     listen_addresses = 'localhost,*'