Additional/altered functionality 

OK, I have made some alterations to this extension, have a look and see if you agree with them:

## Alter background-image-hdpi() and inline-background-image-hdpi()
Now called `background-hdpi()` and `inline-background-hdpi()` receptively.

These mixins now serve as an HDPI option to the entire family of background properties instead of just the image. My reasoning for this is that I think it's more common to use the css `background` property than it is to use `background-image`.

A user can now set all of these properties in one statement, for example:

    @include background-hdpi(icons/logo.png, icons/logo@2x.png, no-repeat top left);
    OR
    @include inline-background-hdpi(icons/logo.png, icons/logo@2x.png, no-repeat top left);

## Added 3 new variables:
### $sprite-hdpi-dimensions: true !default;
You can disable the dimension styles by default when using `sprite-hdpi()` by setting the `$background-hdpi-dimensions` variable to `false` (default to `true`)

### $sprite-hdpi-set-background: false !default;
You can set the background image by default when using `sprite-hdpi()` by setting the `$sprite-hdpi-set-background` variable to `true` (default to `false`)

### $sprite-hdpi-dimensions: true !default;
You can set the dimension styles by default when using `background-hdpi()` and `inline-background-hdpi()` by setting the `$background-hdpi-dimensions` variable to `true` (default to `false`)

As a part of this change I have also set the `$sprite-hdpi-dimensions` to `true` by default. My reasoning for this is I think that if you are using a sprite, chances are you need the dimensions set as well or you will see the other sprites in the sprite map.