[dom] Change DOMTokenList's add() return value. (#114)

As [suggested](https://github.com/whatwg/dom/issues/111#issuecomment-158411312) by @bzbarsky:

Change the return value of add() to have three states: "supported", "not
supported" and "no supported tokens".

The "validation steps" algorithm return values also change accordingly.

/cc @foolip @zcorpan @annevk 
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/114

-- Commit Summary --

  * Change DOMTokenList's add() return value.

-- File Changes --

    M dom.bs (24)
    M dom.html (24)

-- Patch Links --

https://github.com/whatwg/dom/pull/114.patch
https://github.com/whatwg/dom/pull/114.diff

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https://github.com/whatwg/dom/pull/114