Add option to optimize the JVM for short-runnning applications

Ideally, dotc should reuse a resident compiler and we should not fork sbt
for every task. Until this happens, this option is useful for
development. Fixes #222.

Usage:
$ sbt -DOshort=""
$ ./bin/dotc -Oshort foo.scala
Merge pull request #234 from smarter/try/jvm-tuning

Add option to optimize the JVM for short-runnning applications