Reimplement walk() funciton outside of ArtifactoryPath

The walk() function doesn't really belong to ArtifactoryPath.
It's better to reimplement it outside, so that it's possible to
apply it to any Path object.
bump version to 0.1.6
Make ArtifactoryPath iterable

Add __iter__ method to ArtifactoryPath.
Solves #6
Merge pull request #7 from Parallels/develop

Add support for iterating over ArtifactoryPath and move walk() outside of ArtifactoryPath