Finish Hotfix-ReleaseManagement, release script takes 2 arguments again and can be called from buildbot directly
Merge branch 'develop'
Merge branch 'develop'
Update buildbot.sh

Fixes issue #300 - buildbot script exits with 1 on git pull failure.
Implemented rhomass_critical function in AbstractState.  Closes https://github.com/CoolProp/CoolProp/issues/303

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Add surface tension and pc to docs - closes https://github.com/CoolProp/CoolProp/issues/314 and https://github.com/CoolProp/CoolProp/issues/302

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>