Modified DEV java settings.

We have a number of different developer machines with varying amounts of memory and such (not to metion different operating systems).

The JVM will now use a percentage of physical memory by default, but it is easy to override options by doing `export FRONTEND_JVM_ARGS='XXXXX'` in for example you `~/.bashrc` file.

JVM options and Java version are now also displayed when starting SBT.