Similar to git, we now ignore options which have no value.
Previously it would not handle it consistently, and throw a parsing
error the first time the cache was built.

Afterwards, it was fully usable though.
Now we specifically check for the case of no-value options instead.

Closes #349

* config parser now handles quoted values correctly. This doesn't hamper
  multi-line support.
* added regression test to travis to assure we will be warned if we
  rewrite and break the user's .gitconfig file
* only rewrite configuration files if we actually called a mutating
  method on the writer. Previously it would always rewrite it.

Fixes #285

Once again, __del__ is not being called as usual anymore in python 3 ... .

A test verifies it's truly working.

Related to #238

We implement it as described in this article:
http://stackoverflow.com/questions/1557183/is-it-possible-to-include-a-file-in-your-gitconfig

Thus we handle

* cycles
* relative and absolute include paths
* write-backs in case of writable GitConfigParser instances

Fixes #201

That way, flake won't have to completely ignore issues it might have
in other portions of this file.

This brings us much closer to what git can do, and should at least prevent errors while reading
configuration files (which would break a lot of features, like handling of remotes since these rely
reading configuration files).

Fixes #112

Kind of tackling the tasks step by step, picking low-hanging fruit first,
or the ones that everyone depends on

Now it's about going through PY3 issues