Lift Framework 3.1.0-M1

Lift 3.1.0-M1 marks a new release process. Now that Lift 3 is out the door
with most foreseeable breaking changes, Lift 3.1.0 will be developed on a
regular cycle, per the discussion @farmdawgnation initiated on the Lift ML
regarding our release cycles.

3.1.0-M1 is a little late by that account as we finalized the new cycle a little
after the planned date called for. In general, expect bimonthly milestone
builds, with 3 milestones before a final minor release. The first RC for a final
minor release will come about 2 weeks after the third milestone release for
that version, with an eye to having the final release out one month after the
last milestone, barring major bugs.

With that, 3.1.0-M1 contains a few things that were pent up during the Lift
3.0 release cycle:

• (#1812, #1815, #1823) Full compatibility with Scala 2.12. Lift 3.0.1 was
  released with Scala 2.12 support, but there were still some pieces to
  put in place for proper continuous integration and release building before
  we could consider that complete. This is now done. Thanks to @SethTisue
  and @farmdawgnation for putting in some contortions to get this all in order.
• (#1813, #1824) Session-aware Lift and Scala Futures. Especially with Lift 3's
  enhanced support for futures in CSS bindings and RestHelper, folks
  would find themselves firing off futures that needed to do i18n or something
  else that required session access, only to have that session context missing
  when the future actually executed. @pdyraga added a new helper,
  LAFutureWithSession.withCurrentSession, aliased as S.sessionFuture,
  which can be used to create an LAFuture task that has access to the
  session. There is a similar helper, FutureWithSession.withCurrentSession,
  that will spawn a task as a Scala Future with access to the session. The
  resulting futures can be chained and combined with other futures and will
  preserve their session access. This includes all APIs in Scala's Future in
  Scala 2.12, as well.
• (#1819) The mongo Java driver has been updated to version 3.4.0.
• (#1817, #1822) lift-json has gained significant parser performance
  improvements, and is now competitive with the jawn parser in most
  of jawn's microbenchmarks, making it one of the fastest Scala JSON
  parsers across a variety of parsing scenarios.