Slimmage Background Image plugin

[protherj]

This is my first attempt at a Background Images plugin based on the Pull Request #37 from dancek. Slimmage has changed considerably and I hope I got it generally right.

Note: see the thread for Pull Request #55 for more background.

[protherj]

How do I fix the Travis CI build issues?

[erikbra]

Looks like performance issues, if I interpret the log correctly (red timings), or am I wrong?

[lilith]

First we need to fix the existing problems. Then we can address this PR by adding associated tests for the new functionality.

[wiedikerli]

@nathanaeljones whats the status of this pr?

[ProNotion]

This is working great for me so far - thanks Jason. The only issue of slight concern is the lack of fallback like we have with elements on images but I'm not sure how we could achieve this with background images in a nice clean way.

[wiedikerli]

@nathanaeljones any plans to merge this pr?

[firthy]

@nathanaeljones I'd like to see this PR as part of the plugin also

[lilith]

Slimmage.js tries to do a very difficult thing. 99% of possible techniques fail on at least 1 crucial browser. This is not a space where I can merge pull requests for features which don't have associated unit tests. The chance of any new feature working across the same set of browsers is miniscule, in general.

I can't merge this without tests, and I don't have the resources to fix the bitrot in the existing tests. If someone wants to tackle #58, I hand out write access quite readily. Once #58 is fixed, then tests can be added to this PR and (if they pass), it can be merged.