Update Mesos to 0.21.0 and add option for pure java API code.

Add a configuration flag that allows switching between using the native library from Mesos and the
jesos pure java API. Default is to still use native code.

This also updates the mesos version to 0.21.0.
Merge pull request #351 from hgschmie/jesos-option

Update Mesos to 0.21.0 and add option for pure java API code.