Pixel ratio tests
Pixel ratio implementation

- added Imager.transforms to let room to customise things further on
- replacing {pixel_ratio} pattern in `data-src` URI
`pixel_ratio` -> `pixelRatio` as method name.

Also made the pixel ratio computation executed once, not every time an image is replaced (because it is not supposed to change within the lifecycle of a page).
Codestyle consistency
Merge pull request #27 from oncletom/feature-pixel-ratio

[READY] `data-src` + pixel ratio variable