Add 'export_image' method to 'ObjectGraph'.
Add tests for explicit and implicit format determination.
Update CHANGES.
Merge from master.
Be strict about writing .DOT file in UTF8.
utf-8 seems to be the more standard name for the encoding.
Avoid the need for a temporary file by sending the graph straight to stdin.  (Thanks, @sjagoe.)
Move export_image functionality to the AnnotatedGraph.
Merge pull request #19 from mdickinson/feature-export-image

Add 'export_image' method to 'ObjectGraph'.