Fixes #21 JS/CSS not ending in a newline errors

YUI Compressor (and probably every other compressor, and runtime) would
break when bundling files that did not end in a newline.

Solution: Make everything end in a newline.
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.
Clean up < Jekyll 2 compatibility