bundle2: very first version of a bundle2 bundler This changeset is the very first of a long series. It create a new bundle2 module and add a simple class that generate and empty bundle2 container. The module is documented with the current state of the implementation. For information about the final goal you may want to consult the mercurial wiki page: http://mercurial.selenic.com/wiki/BundleFormat2 The documentation of the module will be updated with later patches adding more and more feature to the format. This patches also introduce a test case. This test case build and use its own small extension that use the new bundle2 module. Since the new format is unable to do anything right now, we could not use real mercurial code to test it. Moreover, some advanced feature of the bundle2 spec will not be used by core mercurial at all. So we need to have them tested.