Add SBT cookbook in the documentation
Various documentation improvements

* Added YourKit link to highlights
* Added note about Scala 2.11 to highlights
* Added note about local ivy repo to highlights and migration
* Rewrote documntation on extending Play
Add temporary override of js engine dependency
Merge pull request #2808 from pvlugter/update-js-engine

Add temporary override of js engine dependency
Configure zentasks sample for less and coffeescript
Configure java/zentasks sample for less and coffeescript
Fix possibly null email field in java/zentasks sample
Configure scala/comet-live-monitoring for less and sbt-web
Merge pull request #2806 from jroper/docs-improvements

Various documentation improvements
specs2 dependencies should be "test" scope
Fix typo in Heroku documentation
Merge pull request #2468 from Mogztter/issues/2408

Add SBT cookbook in the documentation
Merge pull request #2818 from OlivierBlanvillain/patch-1

Fix typo in Heroku documentation
Update js engine for longer timeout and improved logging
Merge pull request #2811 from xuwei-k/specs2-dependency-scope

specs2 dependencies should be "test" scope
Merge pull request #2804 from pvlugter/zentasks-assets

Configure zentasks sample for less and coffeescript