rewrite dostring folding

[drdv]

PR Summary

Following #1774 and the modifications proposed in #1776, an alternative structure of the docstring folding code is suggested. The new implementation handles:

• module-level docstrings
• class docstrings
• docstrings with both " and ' delimiters
• comments (containing " and ') between function signature and its docstring

For each toggling, the function python-info-docstring-p is used only once. Two new unit tests have been added.

This PR was originally submitted at galaunay#6 (and later on moved here).

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 (-3.2%) to 88.671% when pulling 22ef412 on drdv:even-better-docstring-folding into fa58b0b on jorgenschaefer:master.

[coveralls]

Coverage remained the same at 91.843% when pulling 22ef412 on drdv:even-better-docstring-folding into fa58b0b on jorgenschaefer:master.

[galaunay]

Sorry for taking so long to have a look at this.
Could I ask you to rebase on the current master branch ?
(This should run the automated tests, which can help)