pylint: Fix a large amount of warnings and errors found by pylint

The errors range from indent errors to unused imports.

There's still plenty of things pylint is complaining about but
on pylint's completely bogus scale this improves the score with
pylint 1.1.0 on my machine from 5.37 to 6.79.

While at it, tune .pylintcfg to ignore more issues:
missing method docstring warnings, longer minimum line lengths etc

I'm looking at doing this since I have a branch where I'm fixing more of dulwich to work on both py2 and py3k and the current pylint issue output is too verbose to see anything useful.