correct initialization of props attribute to object attribute

by defaulting to None rather than {}, the props attribute becomes a
object attribute rather than a (shared) class attribute.
unit test showing the behavior of props dicts in Species

the props attribute should always be a local object attribute,
never shared among other instances.
Rename a couple of unit tests

The first line of the test docstring ends up in the unit test nose log,
so it's good to be succinct.
Merge branch 'pr/293' to fix Species.props dictionary.

Closes #292 and #293