Asset serving

[noelwelsh]

In development mode we don't want the web browser to cache assets, such as JS or CSS, that are under development. This PR implements this feature, which consists of a type of Route for managing assets. It:

• sets up a directory watcher for a given directory, calculating hashes for each file in the directory and recalculating the hash on change.
• implements pathTo by replaces file names with the file name + hash
• serves files by removing the hash and serving the underlying file