Work on SPACK-41: Optional dependencies work for simple conditions.

- Can depend conditionally based on variant, compiler, arch, deps, etc
  - normalize() is not iterative yet: no chaining depends_ons
  - really need a SAT solver, but iterative will at least handle
    simple cases.

- Added "strict" option to Spec.satisfies()
  - strict checks that ALL of other's constraints are met (not just
    the ones self shares)
  - Consider splitting these out into two methods: could_satisfy() and
    satisfies()
    - didn't do this yet as it would require changing code that uses
      satisfies()

- Changed semantics of __contains__ to use strict satisfaction (SPACK-56)

- Added tests for optional dependencies.

- The constrain() method on Specs, compilers, versions, etc. now
  returns whether the spec changed as a result of the call.