Update commons to use released pants.
Update commons to use released pants.

Update pants script to use a virtualenv-installed pants from pypi.
Also, update installation and release guides.

This change sets commons up to use the released pants, which will
be necessary after the pants code moves to pantsbuild.

(sapling split of 3e6bf33c932b4a7c0ab2bc401449387ac84ef1bc)
kwlzn/pex,pombredanne/pex,sixninetynine/pex,kwlzn/pex,rouge8/pex,rouge8/pex,mzdanieltest/pex,jamesbroadhead/pex,pombredanne/pex,pantsbuild/pex,URXtech/pex,jamesbroadhead/pex,snyaggarwal/pex,sixninetynine/pex,mzdanieltest/pex,jsirois/pex,URXtech/pex,snyaggarwal/pex,pantsbuild/pex,jsirois/pex