Added more types of constraints and a way to loop trough them.
All constraints are now named "EnsureSometing".
NF: Validation is now performed via AndConstrainer and OrConstrainer classes,
which allow and/or combination of parameter constraints.
AndConstrainer and OrConstrainer are now callables. Examples have been added.
NF+BF Constraints are now represented completely and correctly in the docstring.
Kill whitespace
Rename constrainer classes and get rid of unecessary iterables
Change handling of None as a Parameter special case
Make parameter constraints public

Will need that for the cmdline interface)
Make the rendered paramter descriptions look nicer/simpler

for simple cases
Remove obsolete special case handling
Support numpy arrays

Same thing should be done for others too. Seems to call for an
intermediate base class (dtype checker). But I am still debating with
myself...
Fix up tests