Add a requirements section to the specification (closes #3, #7, #13, #14)

Add a requirements section to the specification. Most requirements were extracted
from the introduction.
Promises are now in EcmaScript 6, not DOM4

Promises are now in EcmaScript 6, not DOM4. Update the specification
accordingly.
Merge pull request #60 from cdumez/promise_es6

Promises are now in EcmaScript 6, not DOM4
Merge pull request #58 from cdumez/requirements_section

Add a requirements section to the specification (closes #3, #7, #13, #14)
Stop referring to "UnknownError" as it is not part of DOM4 (closes #28)

Stop referring to "UnknownError" as it is not part of DOM4. Also use
DOMException instead of DOMError which is deprecated.