save method does not respect base_path

[dahlbaek]

For instances of the Package and Resource classes, the save method does not take into account the base_path argument of the instance when saving to json. From the documentation, I would expect the target argument to contain a relative path (tested with the helpers.is_safe_path function), to be appended to base_path.

If you would like, I could try my hand at a PR.

[roll]

Switching to a feature label.

Proposed design:

package.save('path.json', to_base_path=True) # better name? relative_to_base_path?