Support 'postgres' protocol in heroku like db urls.

Heroku like db urls were supported with 'postgresql' protocol.
In my heroku app, the `$DATABASE_URL` was set with 'postgres' protocol,
so that the db url could not be parsed:

    2013-07-01T11:51:41.448436+00:00 app[web.1]: [info] application - $DATABASE_URL: postgres://xyz:xyz@ec2-xx-xx-xx-xx.compute-1.amazonaws.com:5432/somedb
    2013-07-01T11:51:41.521684+00:00 app[web.1]: java.util.NoSuchElementException: key not found: host
    2013-07-01T11:51:41.517811+00:00 app[web.1]: Oops, cannot start the server.
    2013-07-01T11:51:41.522280+00:00 app[web.1]:    at scala.collection.MapLike$class.apply(MapLike.scala:141)
    2013-07-01T11:51:41.522379+00:00 app[web.1]:    at scala.collection.AbstractMap.apply(Map.scala:58)
    2013-07-01T11:51:41.523042+00:00 app[web.1]:    at play.api.Play$$anonfun$start$1.apply$mcV$sp(Play.scala:63)
    2013-07-01T11:51:41.522541+00:00 app[web.1]:    at Global$.getDbConfig(Global.scala:42)
    2013-07-01T11:51:41.522458+00:00 app[web.1]:    at com.github.mauricio.async.db.postgresql.util.URLParser$.parse(URLParser.scala:45)

This change supports both 'postgresql' and 'postgres' protocols.