[Lift 2.6] 2.11 compatibility stuff

Branched off of the significant work @fmpwizard had already done, pulled in some
deprecation fixes from `deprecator-exterminator`, fixed all specs2 2.x-related test
failures, and added a compatibility layer that's necessary to deal with some
`concurrent.Map`/`ConcurrentMap` shenanigans Scala pulled over the course of
the 2.10->2.11 cycle.

We now have two projects, `lift-framework` for building 2.11 and ostensibly 2.10.x (I
haven't checked the latter) and `lift-framework-pre-211` to build 2.9.2 and 2.10.x.

All specs are passing under 2.9.2 and 2.10.4 for `lift-framework-pre-211` and all specs
are passing under 2.11 for `lift-framework`. Would appreciate it if folks could test-drive
a local publish of this to make sure their apps are working as expected as well.

We also need to discuss if we're going to backport this stuff to 2.5 or say that you have
to use a 2.6 milestone to get 2.11 support.