[doc] More WS documentation, worked examples

Need to do some follow up work on http://www.playframework.com/documentation/2.2-SNAPSHOT/ScalaWS to match http://www.playframework.com/documentation/2.2-SNAPSHOT/JavaWS and provide more examples for both JavaWS and ScalaWS (some work already done in #1475).

https://groups.google.com/forum/#!searchin/play-framework/WS/play-framework/UylCl0mAa00/Wyq4_uUt9cEJ
https://groups.google.com/forum/#!searchin/play-framework/WS/play-framework/BYeyBuannGs/oxdssGDoVOkJ

Examples:

* Using recover
* Using for comprehensions for futures
* Using withAuth()
* Providing toJson and toXML formatting.