Remove build-time dependency on numpy

This PR implements *Option 2* as suggested in #81.

Short summary:

- need only one build per ``#(architectures) * #(python-versions)``
- `setup.py` becomes simpler (removing complexity due to numpy build time
  dependency)
- rather small code modifications
- there should be no measurable performance impact (though, this yet
  remains to be tested!)
- removes build time warnings regarding deprecated numpy API