Support Python 3k

This is an attempt to support Python 3k in biggles (see #42). This new branch has been rebased on master, so it includes the changes made by @esheldon with pull #51.

Here is a list of tasks to complete before this branch can be merged:
- [ ] Fix the segfault with `HammerAitoffPlot` plot (see [example7.py](https://github.com/nolta/biggles/tree/master/examples/example7.py))
- [ ] Fix the segfault with `Contours` plot (see [example8.py](https://github.com/nolta/biggles/tree/master/examples/example8.py))
- [ ] Fix the module initialization with Python 2.x
Should we have #ifdef guards around this?  I don't think PyBytes exists in older python.