Support of relative image sources

If an File-based image is not placed correctly, embedding the image will fail. This sanity check creates a relative path to the HTML IO object.

Pre-conditions:
- The image to be embedded must be file based
- The html IO object must be support an absolute_path method (http://www.ruby-doc.org/core-2.1.2/File.html#method-c-absolute_path)
Correct the calculation of relative paths in the html reports

Add spec for the conversion to relative paths in the html formatter.
Merge PR #697 "Support of relative image sources". Update History.md.