Added omnia metapackage, fixes to msmbuilder.

@rmcgibbo  I pulled the explicit numpy prequiquisite from msmbuilder as a way to avoid building a giant grid of numpy versions.  Sound reasonable to you, given that all the heavy numpy work happens upstream in either fastcluster or MDTraj?

I've tested the skeleton omnia-md recipe with a fresh miniconda:

```
~/miniconda/bin/conda install -c https://conda.binstar.org/kyleabeauchamp omnia-md
```