Merge pull request #2 from pvlugter/master

Add remaining twirl parts
Made rendered templates immutable

Fixes https://github.com/playframework/playframework/issues/2676

This makes rendered templates use immutable.Seq instead of
TraversableOnce.  See above issue for an extensive discussion of the
implications of doing this.
Merge pull request #3 from jroper/immutable-templates

Made rendered templates immutable