Preparing for the use of git.
Before building, we need to check out the latest stable version of
intermediate XEPs. The Python git module can easilly retrieve older versions
as string. So I change the build process to work from a string containing the
raw XML.
Also preparing the init of the XEP-object to make it possible to update
the raw xml and reread the XEP.
Also improving the use of temporary directories a bit: standard build in the
(temp) directory that was created when initating the repository.
Finally some minor improvements to docstrings etc.
Add reverting (with git) interim XEPs
Do not use the python git module: it is utterly broken. And Dulwich is nice
but its API is quite clumsy (sorry Jelmer). In the end it is much easier to
call 'git' as a subprocess.