Add Debian postinst script to create /etc/dmedia/machine.json

dmedia creates a 'dmedia/machine' doc to track each machine in a group of peers sharing the same dmedia library.

Currently dmedia stores this in CouchDB only and uses a special dmedia/_local/machine doc so it knows which one this machine is. But it would be better to have this in /etc/dmedia/machine.json so if you happen to delete all your user CouchDB files, you'll get a stable machine id when you use dmedia next. Also nice because if multiple users share a computer, they get the same machine id.

We should probably have a /var/lib/dmedia/create-machine script that a Debian postinst script calls.

Here's how to create the machine doc:

>>> from dmedia import schema
>>> schema.create_machine()
{'type': 'dmedia/machine', '_id': 'KY7WRUNJCEAXQKQI7QB2UV2T', 'ver': 0, 'hostname': 'jgd-test', 'time': 1316661512.761287}

Note that the doc saved in /etc/dmedia/machine.json wont have a CouchDB doc['_rev'], so it can be saved into CouchDB without conflict as long as that doc['_id'] doesn't exist.

The idea is we pass this to dmedia.core. As a fallback, if /etc/dmedia/machine.json doesn't exist, we should look for ~/.config/dmedia/machine.json and create it if it doesn't exist. Also makes things a bit friendlier for running dmedia in-tree.

I believe all postinst scripts need to be shell scripts. Relevant Debian Policy:

http://www.debian.org/doc/debian-policy/ch-maintainerscripts.html

The postinst scripts in apache2 seems like useful reference examples.