Embedded static subsite

Here is the first piece of the embedded static subsite, which I think is ready for merging unless there are more comments about the API.  I split it into three commits to hopefully ease reviewing.  The first commit includes the subsite itself plus some tests but no generators.  You can compile and test just this commit by itself.  Next, I add all the generators and tests for the generators in another commit.  Finally, the last commit updates the sample file and some comment updates.

The haddock after all three commits is http://www.math.uic.edu/~lenz/yesod-static/

What is currently missing is the javascript label stuff (an old version is on my master branch).  I plan on working that into a commit on top of these three soon.  Also, the autosprite stuff has not been updated to this new API yet either.  That we can decide if we want to stick right into yesod-static or in a new module (the autosprite stuff adds more dependencies like on JuicyPixels).