Refactor Build.File using monad transformers
Recompile modules when their dependencies have changed.
Style fixes and -Walls for a couple files.
experimental support for setting favicons

Works in Chrome and FF for sure. Safari definitely does not work. Not
sure about Opera and IE9+
Merge in @maxsnew's fixes from #424

Tested on small file where I changed a dependency and on elm-lang.org
code.
Improve error messages as described in a list discussion

https://groups.google.com/forum/#!topic/elm-discuss/X4V4edxeFKI