Define rapi_error_t, rather than using a bare int for error codes
Use typemap to check error code
more comments and debug output
Fix error return type
Define exception throwing code for functions indexing contig list
Index contig list with int instead of size_t

If we use a size_t, passing a negative index results in an
"OverflowError" being thrown as the number is converted into
size_t by SWIG. I'd prefer that an IndexError be thrown, as it is with
this commit.
More unit tests

This commit also breaks up the various unit tests into various
unittest.TestCase classes (still in the same module though).
Rename rapi.batch_wrap to rapi.read_batch