Rewrite of fetching and processing (closes #17, #186, #188, #195, #198 )

* Formalizes using link element as the solution (closes #17)
* Banned media attribute. At least, we say that it's not considered
during processing. (closes #186)
* Added text about when events need to be fired (closes #188)
* The fetch part of the specification now mentions the crossorigin
attribute (closes #195)
* fixed copy/pasta error relating to icons/orientation (closes #198)
* fixed a whole bunch of editorial issues.
Fixes based on @kenchris's feedback

* various fixes suggest by @kenchris
* moved icon and name fallback behaviour out of processing section.
Merge pull request #203 from w3c/cors

Rewrite of fetching and processing (closes #17, #186, #188, #195, #198)