Add density option; improve tests & documentation

[mbklein]

This PR has three distinct commits in order to make review easier, but they should be squashed on commit.

The new feature is the addition of a density option to the constructor, which tells the processor to assert a given pixel density (in pixels per inch) in the output image. This has absolutely no effect on the size or quality of the image; it's simply a convenience so that images show up at a reasonable size when imported into Photoshop, Illustrator, or InDesign.

In adding this option, I also refactored the new Processor() constructor so that it can take two different forms:

The old way (for backward compatibility; will give a deprecation warning):

new Processor(url, streamResolver, [dimensionFunction], [maxWidth], [includeMetadata])

The new way (using an options object for optional values):

new Processor(url, streamResolver, { maxWidth: 1024, density: 600 })

Additional changes:

• Bump version to 1.0.0 because of the constructor interface change
• Add 40 additional tests, bringing coverage from 57% to nearly 100%
• Improve documentation
• Remove .gif as a target format, because it turns out VIPS doesn't support it natively and it never actually worked

[mbklein]

And after all that, I forgot my own admonition and merged without squashing. Ah well.