Added ability to set NodeSource repository's Pin-Priority.

This is very useful on systems where apt is configured with multiple
repositories with different Pin-Priority settings.
A user can set the NodeSource repository's Pin-Priority to a higher number
than, for example, the Pin-Priority of stable-backports repository to prefer
installing Node.js from NodeSource rather than stable-backports.
Merge pull request #3 from tjanez/apt-repo-pin-priority

Added ability to set NodeSource repository's Pin-Priority.