Make sure there aren't multiple solutions in the resolve tests
Don't install a package with a feature unless it is explicitly requested

This will prevent mkl versions of numpy, scipy, etc. from slipping in when
they weren't asked for.

In general, the use-cases we have in mind for features are such that they
shouldn't be installed unless they are specifically requested.

Fixes #629.
Merge branch 'master' of github.com:conda/conda