Add support for specifying parent container

This PR addresses @rstacruz' comments from #33 and updated it to work without the jQuery dependency. 

Good idea on adding a conditional class to avoid having to adjust css values. 

I opted to keep the `parent` option because I feel code ought to be self-documenting, and one of the first things I look for when reading a piece of code is look for the options hash. Having the default just be 'body' specified in string form implies you just need a CSS selector. I think atleast :smile:, I don't mind removing it if you'd prefer though.