Add dpr setting to processImages transform

Relates to https://github.com/assetgraph/assetgraph-sprite/pull/13#issuecomment-68345087

Add support for setting the device pixel ratio of the image asset that is the result of a `processImage` run.
The idea here is to be able to set the device pixel ratio as part of the image pipeline without having to set the device pixel ration in the filename itself, given that the file name cannot contain the `@2x` syntax if the image pipeline is what creates the image.

I've changed the behavior so `dpr=/\d+(?:[,\.]\d+)?/` gets removed from the url during the transform, and the corresponding device pixel ratio is applied to the resulting new asset or the existing asset if no new one is created.

This means that more of the code in the transform is now run than before, since the whole used/unused query string logic was only in use if filters were applied. Now these parts are in play for every asset.

@papandreou what do you think of this?

Ping @silvenon