Get jenkins install script into CKAN core

[seanh]

After checking out a commit to test from CKAN's GitHub?, Jenkins runs a script that creates a new virtual environment and installs CKAN and its dependencies into it, and does some other necessary tasks. Jenkins then runs the tests in this virtualenv.

The install script may have to change from one commit to the next as CKAN's install instructions change, so it would be good if the script was shipped in CKAN core. That way Jenkins will run different versions of the script depending on which commit it's testing and if the tests fail because the script is wrong then that's actually a bug that needs to be fixed in CKAN core. Also the CKAN install instructions could be simplified a lot by just having users run this script instead of doing each step by hand.