Update "Building From Source" to 2.2-SNAPSHOT

Remove the references to old SBT version.
Remove the reference to Play20.wiki.git (documentation is inline).
Change references to "Play 2.0" to be just "Play".
Fix the SBT plugin to say 2.2-SNAPSHOT
Remove link to invalid stack overflow question.
Remove reference to "Play Beta and edge"
Use "active voice" to keep headers consistent.
Remove period at the end of header.
Merge pull request #1312 from wsargent/patch-2

Update "Building From Source" to 2.2-SNAPSHOT