Option to stop redundant download of small images.

This option stops Imager.js downloading a new image when a higher
resolution copy has already been downloaded. This can occur when a
mobile device is rotated from landscape to portrait.

Upscaling to larger images is unchanged.
Make default behaviour following discussion at #77

And fixes 'strict' warning for getNaturalWidth definition.
Fix jshint complaints #77
Porting `determineAppropriateResolution` to a dedicated section.

- splitting the test case for more granularity and TDD conformance
Moved the `availableWidths` to `html-options`.

- it is where the other existing alternatives are tested
- this is where the natural width test is going to be written
Forwarding `data-width` and adjusting responsive image size.
Updated CHANGELOG
Updated CHANGELOG
Updated CHANGELOG