accurate YouTube play button

[chuanqisun]

I saw the comment in your css /* TODO: Consider replacing this with YT's actual svg. Eh. */, so I went ahead and implemented it to address #16

Some trade-offs to consider with this change:

1. The button will look identical to YouTube's, convincing user that this is not a phishing website
2. The CSS file size got reduced
3. The HTML file size increased
4. The pro-usage snippet is longer

Feel free to reject this PR if this is not the direction you want to take this project it. It was a very quick job for me so don't worry about it.

[chuanqisun]

fyi, this will cause some conflict with #8.

[paulirish]

i'd feel a bit better if we could use the SVG from CSS so the progressive enhancement story still has very clean and minimal HTML

does using the SVG as a background image present problems?

[chuanqisun]

Progressive enhancement is a good point. Let me try the background image approach while resolving the conflicts.

[paulirish]

@chuanqisun thanks again for the contribution. i merged something quite similar in #39

cheers