Backport macros to support Scala 2.10.x on master branch

This adds support for Scala 2.10 without any modification to the API or specs.  This will enable libraries that migrate to scala-logging-2.x to continue to cross build for Scala 2.10 and 2.11 without conditionals in the build or creating their own copy of the `Logging` trait.

Relates to #1, #4, #13