Fixes #23 Missing `relative_path` method
Fixes #23 Missing `relative_path` method

Changes in Jekyll 2 to the document objects have introduced new methods
and behavior.  My impersonation of a `StaticFile` failed in Jekyll 2
because of this missing method.

This fix worked in my testing, but may crop up again when the behavior
changes.

moshen/jekyll-asset_bundler,episource/jekyll-asset_bundler