Support Python 3k

This is attempt to support Python 3k in biggles (see #42). Note that the branch is **not ready to be merged**. The purpose of this pull request is to make the code review easier.

Here is a list of tasks to complete before this branch can be merged:
- [x] Fix the segfault at run time: done by @beckermr in 12b18201bf9114a6a6f5b9c264aaf61a01736f3d but @esheldon plans on implementing something better
- [x] ~~Allow the examples to be run in the example directory~~: not a Python 3k specific issue, see #47 
- [ ] 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))
- [ ] Restore the capabilities removed by commit 11e6feda5e03eec297f3a0e2777af609ebb8ce5f