[ci skip]

Artifacts metadata field will be used to store a filename of gzipped
file containing metadata definition for given artifacts archive.

This is needed because of backward compatibility. Previously artifacts
archive had `.tar.gz` format, but artifacts browser requires ZIP format
now.

`artifacts?` method checks if artifacts archive is available.

This support is not completed though, as parent directory that is first
in collection returned by `directories!` is not iterable yet.

This implementation makes it possible to browse artifacts, it
depends on artifacts metadata.

`StringPath` class is something similar to Ruby's `Pathname` class,
but does not involve any IO operations. `StringPath` objects require
passing string representation of path, and array of paths that
represents universe to constructor to be intantiated.

This reverts nesting artifacts controller in builds module.