Allow source and target versions to be specified
Allow source and target versions to be specified

* Many additional options can be specified for javac, but this won't be
  implemented for this version of rake-compiler.

Signed-off-by: Alex Coles <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@alexcolesportfolio.com>

cosmo0920/rake-compiler,Wirachmat/rake-compiler,rake-compiler/rake-compiler,larskanis/rake-compiler