ENH: Automatic build of input and output ports for VisTrails

- Function added: define_output_ports(docstring)
    Automatically constructs a list of output ports of VisTrails
    OPort objects based on scraped docstring
- Function added: define_input_ports(docstring)
    Automatically constructs a list of output ports of VisTrails
    IPort objects based on scraped docstring.  If parameter
    is optional in the module docstring, the input port will
    also be optional.
- Dictionary added: sig_map
    Maps from one VisTrail port signature to many python/numpy
    types that might be represented in the docstring.
- Function added: get_signature(arg_type)
    Loop over the sig_map to try to map from docstring parameter
    type to a VisTrail port signature.  If there is not a mapping
    an exception will be raised.