Feature: use jedi for `definition_location` instead of rope

[asfaltboy]

For me, the main reason for this change is that Jedi has support for following flask.ext imports.

Jedi's functionality differs from rope in (at least) one noticeable way:

• Rope immediately follows a symbol through an import if no assignment exists in current source
• Jedi has 2 different methods documented in it's API:
  • goto_assignments that resolves until assignment (never auto-following imports out of current file)
  • goto_definition which does auto-follow, but always until the very end (up to a module/class/function definition).

I tried both, and goto_assignments feels more natural to me, since it stops on each assignment/import, and I can choose to keep following or not.

I've made the method settable in settings with all 3 options: rope/jedi-assignemnts/jedi-definitions.

I kept the previous rope as the default behaviour, but I would advocate changing the default to jedi-assignments. While this may surprise current SublimePythonIDE users since it will now require another click each time an import statement is reached, I believe this explicit behavior is more correct and that using Jedi outweighs the "new interface learning curve".

@JulianEberius Please let me know how you feel, and I'll make adjustments accordingly.

[asfaltboy]

@JulianEberius bump?