add support for browsers that do not support devicePixelRatio but support media queries in javascript
Fixed strange space characters on line 8.
Only use the @2x image if its content-type is some type of image.
add `XMLHttpRequest.getResponseHeader` to test fixtures.

tests now pass again.
test for proper content-type
allow for default options to be set in one place.

included a function to manually set those options without changing the
source, but this is just useful for testing. tests are included.

Ultimately I think having to explicitly call a `new RetinaSwap( /*
options */ )` or `Retina.load( /* options */ )` is the way to go _if_
configuration is commonly needed, but that requires a larger change and
I'm not sure it is desired.

No matter what strategy is adopted, this may at least provide the
underlying functionality.

@caseyohara is this headed in the right direction?
Save and call any existing window.onload handler
Add tests for pull #48
Delete the existing global document
Merge branch 'check-content-type' of git://github.com/zever/retinajs into zever-check-content-type
Move RetinaImagePath.set_options to #configure() method on Retina object, provides an interface for overriding default configuration. Update tests.
Add additional media queries for pull #42
Supports data-at2x attributes.

Uses supplied data-at2x attributes for the @2x asset URLs instead of doing HEAD requests.
Added unit and functional tests for data-at2x attributes support
Added data-at2x support and hints about Rails usage to README.
Clarified asset digest a bit in README.