New gml reader and writer

This idea is born from issue #972 and the discussion followed suit. This push aims to replace the old GML file format parser and generator with one stricter, faster and less buggy.

- it doesn't have external dependencies
- supports attribute lists – unlike what the `write_gml` documentation says, GML supports list attributes as long as they have at least two elements.
- ensures node ids are integers, if not sets the label as the old id value and assigns an increasing number as id.
- it's more tested.

I'm naturally opened to discussion and modifications.

Luca :)

IronPython 2.7.5 Beta 2.

I reduced the test case to

```python
# coding: utf-8
from __future__ import unicode_literals
from nose.tools import assert_equal

assert_equal(1, len('©'))
```

`ipy test_gml.py` runs, but `ipy -c 'from nose import main; main()' test_gml.py` fails. This is crazy. Let's mark this as a known failure and not waste time on it.