Initial Support for APT repos

Can now sync remote apt repositories thanks to the help of apt-mirror (https://github.com/apt-mirror/apt-mirror).

When creating your repo definition in the config it should look like this:

```
  repositories:
    linuxmint-qiana-main:
      type: apt
      source: 'deb http://mirror.optusnet.com.au/linuxmint/packages/ qiana main'
```

The source is basically exactly the way you would specify them in the sources.list file on an APT machine.

You can either specify multiple "components" (e.g. main, non-free, etc) and have them all pulled in to the one Git managed repo, eg:

```
  repositories:
    linuxmint-qiana:
      type: apt
      source: 'deb http://mirror.optusnet.com.au/linuxmint/packages/ qiana main non-free'
```

Or you can separate them so that you can version control the components independantly, eg:

```
  repositories:
    linuxmint-qiana-main:
      type: apt
      source: 'deb http://mirror.optusnet.com.au/linuxmint/packages/ qiana main'
    linuxmint-qiana-non-free:
      type: apt
      source: 'deb http://mirror.optusnet.com.au/linuxmint/packages/ qiana non-free'
```

Note though that these are 2 different repositories, with different names, and so on the clients it will have to be configured like:

deb http://reposerver/linuxmint-qiana-main qiana main
deb http://reposerver/linuxmint-qiana-non-free qiana non-free