Move globals to function; version_sanitized global
Egg compatibility improvements for Mesos 0.20.0

* Put all the eggs in target
* Conditional behavior based on version of Mesos
Various version handling improvements

* Update version documentation
* Extract nominal-version from source by default
* Move globals to function
* Error handling if version does not match expectations
Fix build failures, must change to mesos-repo

When called with `checkout go`, the go function is executed within the
mesos-repo. In order to call `go` independently, we must first change to
the mesos-repo.
Missing '{'