Environment variable to add java opts to the WinstoneConstroller

To enable testers to add for example heap and perm gen settings to the java process
Ex:
export WINSTONE_JAVA_OPTS="-Xmx3072m -XX:MaxPermSize=1024m"
Rename to JENKINS_JAVA_OPTS
Documentation
Merge pull request #19 from jenkinsci/javaopts

Environment variable to add java opts to the WinstoneConstroller