add the ability to unload images after they have scrolled out of view
add support for defining top and bottom support independently
add a placeholder param
add some documentation to README.md
fix documentation error
cache the placeholder on the element
remove placeholder from README.md
Merge remote-tracking branch 'upstream/master' into unload_on_scrollout

Conflicts:
	README.md
	dist/echo.js
	dist/echo.min.js
	src/echo.js
Merge pull request #39 from raphaeleidus/unload_on_scrollout

Unload on scrollout