[feature] Hardware ID / serial number

[nemesifier]

We should add a new configuration option like "serial_number" or "serial_number_script", which is a path to a script which if exists will be used to retrieve the serial number, which will then be sent to ow2 during registration.

We could add another boolean config option like "serial_number_key" which if true uses the serial number to generate the device key.
The default value of this config option could be true if "serial_number_script" is defined and exists, false otherwise.
It would basically mean the key is generated from the serial number by default when this feature is used, but it can be turned off if needed.

The actual names of the config options may be different than those proposed here if we find better ones.

[okraits]

I'm proposing hardware_id instead of serial_number as the common part in the names of the configuration options as it is more generic. I guess that anybody who wants to use that field for something else than a serial number would be irritated by that name.

[nemesifier]

@okraits great, that's fine for me, we will just have to indicate both namings in the README to help users find this feature more quickly if they need it

[okraits]

@nemesisdesign I already updated README.md in #75 to explain the feature. Please check if the explanation is sufficient. Maybe it would make sense to add a link to the README.md of django-netjsonconfig for additional explanations?

[nemesifier]

@okraits adding a link to the hardware-id README section of django-netjsonconfig is a great idea!

[okraits]

@nemesisdesign I rebased against master and added the link.