support multiple versions of installed libraries

[kyleburton]

bake_require supports pulling remote git repositories at a specific tag, version or branch. In the case that multiple projects are referencing the same remote library, though with different tags, the current implementation doesn't address this situation. bake caches libraries, so the first one wins on first install, although bake update will pull the latest for these, also causing undesirable skew between projects.

Possible solutions are to include the branch/tag/sha in the file system path (choosing master as a default):

~/.bake/packages/<host>/<user>/<repo>/<branch|tag|sha>/

Then bake update would be a no-op for tags and sha's and pull latest for a branch.