Python 3 support

There's still some test failures (in both Python 2 and 3) which I've outlined in `test-failures.txt`. Basically four things need to be decided:

1. Whether or not to drop Python 2.6 support since ReportLab 3 only supports down to 2.7.
2. Try and figure out what is causing the keepinframe test to fail. This is a long standing bug so not that urgent to fix.
3. Fix the PyPDF2 related failures in Python 3.
4. What to do about a ReportLab 3 font loading bug in Python 3.

More info in `test-failures.txt`. I will also add some comments.
This `{posargs}` means that tox can pass arguments through to the test runner using `--`, e.g.:

    tox -e py27,py34 -- -x -t .*tag.*

But while the extra options are detected by `get_options()`, `find_suites()` doesn't seem to listen to them.

It would be nice if things like failfast and pattern filtering worked from tox.