Add a nix build expression, for development

This is a nix expression to build Synfig from source on any distro
where nix is installed. This will install all the dependencies from
the nixpkgs repo and build Synfig without interfering with the host
distro.

http://nixos.org/nix/

To quickly install nix, you can run the following command:

$ curl -L http://git.io/nix-install.sh | bash

To initialise it:

$ source ~/.nix-profile/etc/profile.d/nix.sh

To build synfig, from the current directory:

$ nix-build

To run the newly compiled synfigstudio:

$ ./result/bin/synfigstudio