Fallback to shell 'help()' for docstring when jedi fails

[galaunay]

PR Summary

Follow #1507.
When jedi fails to provide documentation, use the shell help() function to try to get the docstring.

PR checklist

Please make sure that the following things have been addressed (and check the relevant checkboxes):

• Commits respect our guidelines
• Tests are passing properly (see here on how to run Elpy's tests)

For new features only:

• Tests has been added to cover the change
• The documentation has been updated

[coveralls]

Coverage decreased (-0.3%) to 85.155% when pulling 925756f on galaunay:Gather-docstring-from-shell into 2d4a968 on jorgenschaefer:master.

[galaunay]

Note to myself:
pydoc seems to be confused from time to time with python2.7.
Maybe we should provide this only for python > 3 ?