Add support for a callback to be fired when images are lazy loaded
Merge pull request #40 from raphaeleidus/callback

Add support for a callback to be fired when images are lazy loaded
Merge remote-tracking branch 'upstream/master' into unload_on_scrollout

Conflicts:
	README.md
	dist/echo.js
	dist/echo.min.js
	src/echo.js
Query for `<img>` elements only

While this is probably a case of micro-optimisation, [there is a performance gain of some 5%](http://jsperf.com/queryselectorall-with-attr-only-and-with-element-type) when querying for `img[data-echo]` as opposed to `[data-echo]` only.
Merge pull request #42 from vzwick/patch-1

Query for <img> elements only, 5% perf gain
Merge pull request #39 from raphaeleidus/unload_on_scrollout

Unload on scrollout