CONTRIBUTING.md: Add link to CLA.
Fix python setup: Report errors.

No error indication was being given if something went wrong while
setting up python.
Fix python setup: Make sure python version on user's path is picked.

Problem:  executable() was detecting python on user's path, but
          system() was executing system-level python.

Solution: Make sure python version on user's path is executed.
          Use exepath() to force system() to do so.
Merge PR #1358 'Fix python setup'
Merge pull request #1364 from fwalch/cla

CONTRIBUTING.md: Add link to CLA.
Fix python setup: Fix issue for users not using python.