Collected subtree updates

These include:
 * a .gittrees file with meta data
 * new sub commands (push-all, pull-all, from-submodule, prune, diff, list)
 * Documentation updates

Contributers from the git log on https://github.com/helmo/git-subtree
bibendi <bibendi@bk.ru>
Carl Fürstenberg <carl@blockmastersecurity.com>
Francesco Delfino <delfinof@gmail.com>
helmo <rink@initfour.nl>
Herman van Rink <rink@initfour.nl>
James Roper <jroper@vz.net>
John Yani <vanuan@gmail.com>
Jonathan 'Wolf' Rentzsch <jwr.git@redshed.net>
mhart <michael@adslot.com>
mhoffman <matt.hoffman@quantumretail.com>
Nate Jones <nate@endot.org>
Paul Cartwright <paul.cartwright@ziilabs.com>
Peter Jaros <pjaros@pivotallabs.com>
rentzsch <jwr.git@redshed.net>
sun <sun@unleashedmind.com>
Thomas Van Doren <thomas@cozi.com>
Vanya at notebook <vanuan@gmail.com>