Ad unit size mapping

Introduces the [size mapping](https://support.google.com/dfp_premium/answer/3423562?hl=en) option for DFP so that ad units may be served based on screen size. Ad units will be defined with a new `size-mapping` attribute rather than specifying the sizes inline and crudely filtering them.

- Deprecates the `dimensions` property
- Deprecates the `id` property
- Deprecates the `context` property
- Introduces the `size-mapping` property
- Automatically JSON encodes the `targeting` property

### We can target biiiiiig screens
![screen shot 2014-05-28 at 13 39 25](https://cloud.githubusercontent.com/assets/271645/3104281/2989a7a6-e665-11e3-9e0f-0a79426bec86.png)

### And liiiiitle screens
![screenshot_2014-05-28-14-30-16](https://cloud.githubusercontent.com/assets/271645/3104873/b1bea39a-e66c-11e3-9793-7f5422413d5e.png)

Tested in Safari on the iPad, Chrome on the Galaxy Tab and Firefox and Chrome on my phone.