Modified DEV java settings.

We have a number of different machines with varying amounts of memory and such.

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.
Yep, we are building in openjdk 7
Make it clear this is not a real secret
Make it clear this is not a real secret
Added warning about starting SBT to README
Updated to latest version of sbt
Detect os and hardwire OSX total memory to 8gb
Merge pull request #2661 from guardian/dev-jvm-settings

Modified DEV java settings.