make existing code Py3 compatible (WIP)

This effectively makes `vsc-base` require Python 2.6, so be careful when merging this in (EasyBuild, which needs `vsc-base` will only move to Python 2.6 early 2015).

Currently WIP, not all unit tests pass yet with Python3 (but they still do using Python2.6).

Maybe this should include a bump to v2.0?
move this to `p2and3` module, code here should be 
```python
from vsc.utils.py2and3 import is3
if is3():
    from collections import MutableMapping
else:
    from UserDict import DictMixin as MutableMapping
```