BMP now uses a reasonable resolution, and customizable using the "dpi" option.
Adds dpi to the Image info dictinoary.
Addes tests
dpi key should only be present when there is resolution info in the BMP header.
Merge pull request #745 from gcq/master

Handle resolution in BMP files
Update CHANGES.rst [ci skip]
Don't print the entire image as bytes on failure
Merge pull request #749 from wiredfool/helper

Helper Tweak