created layer package

moved layer handling to separate package and refactored `conair build`
and the nspawn package.
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
Merge pull request #2 from teemow/build-container-refactor

refactor `conair build` to create images with layers