Allow arbitrary JVM options to be passed to marathon

The current start script only passes -D options as a command line parameter
to the JVM. This makes it impossible to use e.g. -agentlib or -agentpath or
the various -X options.

This change allows using an environment variable (JVM_OPTS) to contain these
options and have them passed directly to the JVM.

E.g. to start marathon with a tuned garbage collector and memory setting:

JVM_OPTS="-Xmx128m -XX:+UseConcMarkSweepGC -XX:+ExplicitGCInvokesConcurrent -XX:+AggressiveOpts -XX:+HeapDumpOnOutOfMemoryError" ./bin/start <marathon options>