Dockerize!

Hi,

I've kept it simple in this case (so no odd env variable parsing) because I more and more tend towards building images including the config. So after this is merged, I'll add it to index.docker.io which will automatically creates 'prometheus/alertmanager' images.
For an actual deployment, you would just build a vendored image on top of it like this:

    FROM prometheus/alertmanager
    ADD alertmanager.conf