charm-helper should have a single unifying command for using it in a declarative way

In addition to the helpful snippets of code, there are common tasks to *all* charms.

One should be able to write an install hook that does something like this:

#!/bin/sh
charm-helper $0

And from that, the single helper is able to inspect declarative bits of the charm and act upon them. So package info would be put into a packages yaml and remote urls to fetch in a 'remote files' yaml section.

This is the same model as debhelper 7's minimal rules set.