[Trac] update on another overwhelmingly popular patch

[Justus Pendleton]

Inspired by Brad Anderson's post :-)....

I've finished up making trac version control agnostic and adding a darcs
backend.  No doubt there are still errors since I haven't done comprehensive
testing but all of the functionality is there and seems to mostly work.  I've
also changed trac-admin to allow creation of non-svn repositories.

The only real "big" change is that revision identifiers can no longer be assumed
to be integers.  This meant the db schema for the revision table changed.  Right
now this takes advantage of the fact that SQLite is typeless.  If trac actually
starts supporting other databases that enforce type checking then it would be
necessary to provide different schemas for different version control backends. 
A few places in the code that cast the rev to an int have been changed to leave
it as a string.  Based on what Christopher Lenz quoted from pypgsql's FAQ this
should work with postgres as well.

And, again, svn access still works.  Although I haven't done comprehensive
testing on that, either.

This was scheduled functionality, albeit for 2.0, but I'm pretty much done with
it.  At least, as done as it is doing to get without feedback ;-).  If trac's
maintainers are interested in merging this functionality into /trunk, I'm happy
to help however I can.  I've also created a unified diff of my changes in case
that makes it easier to see what I've done.

unified diff of changes: http://www.ryoohki.net/~justus/trac_diff.txt
darcs repository: http://www.ryoohki.net/~justus/trac
running tracd: http://www.ryoohki.net:8080/env