Merge pull request #1 from teemow/delete-graph-layers

Delete unreferenced layers if an image is removed
version: bump to v0.2.1+git
Merge branch 'master' into build-container-refactor

* master:
  version: bump to v0.2.1+git
respect file changes when running `make`
fixed layer package

 * fixed visibility in layer package
 * correct handling if layer already exists