Mesos registry defaults (0.19+) and vercomp

Change the Mesos 0.19+ defaults to:

  --work_dir=/var/lib/mesos
  --quorum=1

This makes it so the package can be installed and used on a single host
without any further configuration.

Add a version comparison function, which allows us to compare two
version strings easily, for example: $(vercomp 0.19 0.2) prints '>'.
Compare all 3 positions, improve function docs

Comparing all 3 positions is a more useful example. Include
documentation about vercomp caveats.
Merge branch 'master' into mesos-registry-defaults
Merge pull request #12 from mesosphere/mesos-registry-defaults

Mesos registry defaults