Add warning when running with TieredStopAtLevel

TieredStopAtLevel is used to enable quick startup of lein, for
example, and disables various levels of JIT compilation, which is
probably not what you intended if you are benchmarking.

This commit will print a warning if ths JVM option is in use.
Updated project.clj, release notes and readme for 0.4.2
Updated project.clj, release notes and readme for 0.4.2
Merge tag '0.4.2' into develop

criterium 0.4.2
Updated version for next release cycle