Start wrapping seq_span

Start wrapping seq_span
there were a couple of challanges. First off the stl_container_adapter I
was using didn't handle returning a const_reference particularly well,
so I had to add a get_const function.

The other was more insideous, the example the wiki had showed just setting
the python exception, it didn't generate a C++ exception. So when
boost::python tried to copy an undefined reference when it fell of
the end of a container I got a bus error.

The solution was to add in a C++/Python exception translater so I threw
a C++ exception indicating an IndexError, but then the it was transformed
into the appropriate Python exception.