Report precent difference when using a tolerance

[ultramiraculous]

When using a tolerance, report the % difference to the console on failure + add a key to the NSError with the percent.

I had to refactor the UIImage categories to separate out the part where we calculate the percent difference. As-written it might be a little faster if the images are the same when using a tolerance, because it tries to do memcmp before it calculates the percent.

Fixes #109

[nscoding]

nice, remove extra line

[nscoding]

so good! Some nit's here and there but thank for working on this.

[ultramiraculous]

Updated the commit w/ the nits taken care of.

[ultramiraculous]

Actually I just realized (by noticing a warning I missed) why that float was initialized. The compiler warns that percentDifference may not be initialized (even though it definitely is) down when we format the error string.

[nscoding]

@ultramiraculous yes you are right, 0 will do right?

[ultramiraculous]

@nscoding, yup I switched it to 0.0 in the last update. Rebasing so the tests pass.

[ghost]

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[ghost]

Thank you for your pull request. As you may know, we require contributors to sign our Contributor License Agreement, and we don't seem to have you on file and listed as active anymore. In order for us to review and merge your code, please email [email protected] with your details so we can update your status.

[ultramiraculous]

@nscoding I completely missed that this PR was still open. You should tell @jamesgpearce the bot is being spammy, but in this case I guess it helped me remember it.

Is it still worthwhile to get this passing?

[ghost]

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