travis: try out hvr's PPA

See http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/20634
use cabal-1.18 instead of 1.16
install newer Haddock, and use multiple cores if available
try factoring before_install out into diagrams-travis repo
specify constraint on haddock version
test whether the script can export env vars
call set_env with 'source'
move more stuff into diagrams-travis
Merge branch 'travis'.  Now getting our travis config & scripts from diagrams-travis repo.