Not Python 2.5 compatible

[benoitbryon]

Requires #26, follows #19.

It almost works in Python 2.5 too: there are a pair of with which require from future import with_statement imports, and 3 str.format() calls which would have to be replaced by %

Support for Python 2.5 is not a high priority as of 2013-05-24: implementation of rst elements is more important. That said, if #26 is implemented, we can work on this ticket.