Add a development Procfile, document in INSTALL

Since introducing Webpack (commit 66f796c0), there is an additional process required to watch and rebuild Angular app assets.

A separate development `Procfile` is one solution to make things work better _out-of-the-box_, although not necessarily the solution recommended by the maintainer of foreman (see https://github.com/ddollar/foreman/issues/358#issuecomment-17556894).
Two questions about the worker process:
* for a developer this process is going to be annoyingly noisy. Should we redirect stdout (i.e. `> /dev/null 2>&1`) or amend the rake task to be quieter by default?
* do we even need this process by default in development mode?