Fix broken output when class docstring is missing

[acarapetis]

Fixes #83.

pyment was generating output with invalid indentation when all of the following conditions were met:

• ignore_private is True (the default on the command line)
• a class has no docstring
• the first method in that class is "private" (i.e. __dunder__) and has a docstring.

When skipping the private method header, the parser still had the waiting_docs flag set, and thus it was mistaking the method docstring for the class docstring.