A PythonRun task.

Brings python running under goal.
Collect python interpreter constraints from all targets.

Not just the python_binary target. This allows specifying the
constraints where they belong - on the targets containing the
code that require them.
Fix missing # coding=utf-8 directives.
Fix scrooge unittest.
Added support for multiple pants_support_baseurls.

Changed pants_support_baseurl to a list. Pants now goes through each url in the list from first to last, downloading the first binary with a valid url (and then breaking). This way, pants users can set their own support repo online and add it to the beginning of the list, so that their repo will be queried before the default repo on pantsbuild.github.io.