New formula: git-archive-all

A python script wrapper for git-archive that archives
a git superproject and its submodules, if it has any.
Takes into account .gitattributes

In the past I've already made a submit, however it was reject. Now, when script popularity is grown, I hope it's notable enough to be added to the Homebrew library.
https://github.com/mxcl/homebrew/issues/2757