Jamroot: make release the default build

* `b2` will now build the release variant, unless you specify `b2 debug` in the command line.
* `b2 test` will still build the debug variant by default and there is no way to run it in release mode (enforced)

fixes #128
Merge pull request #139 from broadinstitute/mc_build_release_by_default

Jamroot: make release the default build