Unify 2.9 and 2.10 templates

Pros:
* transparently handles cross build strategy from 2.9 to 2.10
* one less template for us to maintain while we're still supporting two Scalas
* embraces *.scala, because most projects outgrow *.sbt anyway
* DRYs up version numbers

Cons:
* bigger
* impacts the docs
* probably not optimal SBT.  I don't know what I'm doing.

Feelings won't be hurt if this is declined.  I pull-requested because I want feedback.