Metapackage (fixes conda/conda#161)

Implements a basic `conda metapackage` command. Fixes https://github.com/conda/conda/issues/161. 

A metapackage is any package that doesn't contain files (other than maybe automatically generated files), just metadata. 

Example usage:

```bash
$ conda metapackage scipy-stack --dependencies python sympy numpy scipy pandas ipython-notebook ipython-qtconsole nose matplotlib  --home http://www.scipy.org/stackspec.html --license BSD-like --summary 'Bare minimum packages to get the "SciPy Stack"'
```