Python 3 support

Hi again!

Here is the Python 3 support. I have tested it on Ubuntu 14.04 with Python 2.5.6, 2.6.9, 2.7.6, 3.1.5, 3.2.5, 3.3.5 and 3.4.0.
On my Ubuntu, I just have to do:
```bash
export INLINE_PYTHON_EXECUTABLE=/usr/bin/python3
```
before calling `perl Makefile.PL` and all goes fine.

Remarks:
- I tried to update the POD with some useful information but I'm not a POD expert... I think you should check my english and see if it's really useful... Moreover, it might be a good idea to talk about the "INLINE_PYTHON_EXECUTABLE". I don't  know...
- Supporting 2.5 and 3.1 in tests at the same time is a nightmare, since:
  - b'...' syntax is not allowed in Python 2.5 *at all* (not compiling)
  - u'...' syntax is not allowed in Python 3.1 *at all* (not compiling)

It's why you can see in tests something like:
```python
        # u'föö' is not a valid syntax in Py3.1
        return {'f\xc3\xb6\xc3\xb6'.decode('utf-8'): 'bar'}
```
- `exec` has changed between 2 and 3, it's why the test 19 seems a little strange (http://stackoverflow.com/questions/6561482)

If something seems strange or inappropriate, let met know! I will do more commit in this pull request if needed.
Hope it helps!

Laurent