Embed example and added some things to the API

People, this is based in the Paddy branch with current master merge in...

I have added an example under bokeh/examples/embed...
It is a super simple Flask app which loads a simple line plot.

If you test it as is... the plot will not be rendered because of the BackTick error in the CDN files...
But if you use the `hosted_file`, instead of `hosted_CDN` pointing to local files living inside /static, it works as expected.

I have modified the API (bokeh/embed) a little bit and probably I am missing a lot of things about the design because I am trying to understand all the possibilities and the usability, and for sure there are some problems... but take a look and let me know...