Add support for CloudSigma datasource

This pull request fixes Issue #184.

This datasource is a little bit different from OpenStack and EC2 by the fact that it doesn't use network in order to fetch anything, but a serial port instead. The reasoning is very simple: It is perfectly fine for a machine in CloudSigma's infrastructure to be withOUT any network interface at all and that shouldn't in any way, shape or form be an issue for the machine.

So, I had to write a library that reads from the serial port and knows the specifics of server context. Called in Cepgo and it's available in our GitHub account: https://github.com/cloudsigma/cepgo

At first wanted to import it and use from there, but I saw that you prefer to have a copy of all third party libraries, so I had to add it there. The biggest issue I had was with the fact that our format is completely different from the OpenStack's metadata which made me to add the IP extraction as a method in each datasource. Later I've realized that was a bad idea and removed it :)

From our side we keep the same functionality as with Canonical's cloudinit so the [documentation about it](http://cloudsigma-docs.readthedocs.org/en/latest/meta.html#server-s-metadata-fields-with-special-meaning) is still up-to-date.
Is there a reason to not to call it cloudsigma and instead "server-context-service" which sounds like a generic standard?