RFC: Add buildExternalNixExpression

My attempt at answering/solving the "upstream nix expressions" question in http://lists.science.uu.nl/pipermail/nix-dev/2014-October/014734.html.

Please have a look and give some feedback :-)

----

This function is for building (external) packages that contain a
default.nix file in their sources.

Example usage:

```
  project-name = buildExternalNixExpression {
    src = fetchurl { ... };
  };
```