Simplify deployment by embedding assets into library.

Hi Johan,

we would like to use `ekg` at Erudify to monitor services deployed in our cloud. However, it's reliance on extra data files makes deploying applications built using it unnecessarily difficult. The attached patch uses the [`file-embed`](http://hackage.haskell.org/package/file-embed) library to embed the necessary assets directly into the `ekg` library at compile time. 

It would be great, if you could change `ekg` to use this scheme. If required, then I can add either a cabal flag that allows to compile without embedding the files, or I can add an additional argument to `startServer`, which allows to specify the assets directory. I guess one would typically use the latter for development of the default monitor or for using a simple custom monitor. However, I do expect that for almost all use cases this direct embedding of the assets is the most convenient option.

What do you think?

best regards,
Simon