Fix address in use errors on osx

OSX users were getting failures when trying to bind more than one client
to the broadcast port. This change sets the REUSEPORT socket option on
platforms where it is available.
Merge pull request #102 from devin-c/address_in_use

Fix address in use errors on OSX
Merge branch 'master' into 2-8-stable
Bump version 2.8.0.beta5