Use a decent default for django settings path

[galaunay]

PR Summary

When using Django, Elpy needs the user to set the environment variable DJANGO_SETTINGS_MODULE to know where the setting file is.
This can be cumbersome, as the user will have to change this environment variable every time he switches projects.

With this PR, Elpy will use the default setting path (gathered from manage.py) if DJANGO_SETTINGS_MODULE is not set.

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)

[galaunay]

@gopar, I tried to make Django slightly easier to use in Elpy with this PR.
But don't use Django a lot, so I am not sure I did it right...
Could you give me you opinion ?

[coveralls]

Coverage remained the same at 92.145% when pulling a7b47f3 on galaunay:Fix1629 into 7e8181d on jorgenschaefer:master.