add flash detect

fixes #48

[Here is the list of plugins that this has been tested successfully](https://gist.github.com/patrickkettner/31e9f6678a124bf97f78). 

The only failure is [SafeBrowser](http://digitalplanet.is/safebrowser.html). It does server side removal of `embed` and `object` elements, but doesn't touch those added by javascript. Even if I could detect this someway, I am not sure if this counts as blocking, anyway.

I have done a large amount of testing across all modern browsers, and ie7-11. If there is any flash block technique that this does not cover that anyone is aware of, please let me know.