Better activate/deactivate logic.

spack activate
  - now activates dependency extensions
  - ensures dependencies are activated in the python installation.
  - -f/--force option still allows the old activate behavior.

spack deactivate
  - checks for dependents before deactivating (like uninstall)
  - deactivate -a/--all <extension> will deactviate a package and ALL
    of its dependency extensions.
  - deactivate -a/--all <extendee> activates all extensions of <extendee>
    e.g.: spack deactivate -a python
  - deactivate -f/--force option allows removing regardless of dependents.
    - deactivate -f can be run EVEN if a package is not activated.
    - allows for clenup of activations gone wrong.