Fix sub-package imports.

This fixes a regression where it doesn't know how to checkout branch and tags for packages imported
that are not the base of a repository.
Merge branch 'master' into fix_import_subpackages

* master:
  ignore .gopack/
  Download transitive dependencies.
  Use a graph to keep and search dependencies.

Conflicts:
	model_test.go
Merge pull request #21 from calavera/fix_import_subpackages

Fix sub-package imports.