Improve SetClass situation. about go-js-dom

[dominikh]

• I'll fix the documentation once I get a chance, thanks
• We'll keep SetClass() around, purely because it's part of the DOM API. We may add documentation to point to Class(), however.
• We will not merge Set and SetString – the implied semantic of SetString is that it allows passing in a space-separated list of tokens (as per the definition of a TokenList), so e.g. SetString("foo bar"). If we had something like Set("foo bar", "baz"), that might lead to confusion. Admittedly, the current Set already suffers that same issue ([]string{"foo bar", "baz"}), but we don't need to make it any worse.

[dmitshur]

I am in agreement with your decision in points two and three for now, you provide convincing arguments.

Point one still needs to be addressed.

[dominikh]

@shurcooL Please review #17