Add separate marker ring for goto-definition calls

It's made to to facilitate popping back to where `C-.` was invoked.
Default keybinding is `M-*`.
Make goto-definition-pop-mark key customizable, bump documentation
Change default key for jedi:goto-definition-pop-marker to "C-,"
Make jedi:goto-definition-push-marker internal function rather than a command
Merge pull request #44 from immerrr/add-separate-marker-ring

Add separate marker ring for goto-definition calls