Added a NOW() helper

After some IRC-discussion with @tgriesser, here's a pull request for solving #310.

It adds a `now()` method to the database client so one can do `knex.client.now()` as a shortcut for `knex.raw('NOW()')` and then get the database specific representation of `NOW()` returned – which means better interoperability.

Includes one DB-specific override, that was brought up as an example in #310, for SQLite3 that replaces `NOW()` with `date('now')` instead.