Making the omni completer more robust

- There was a bug in not calling int() on the result of the first call to the
omnifunc.
- We need to be more resilient to badly written omnifuncs and check that the
result of the second call is a list or a dict with a list, as the vim docs say
the omnifunc should return on the second call.

One of the two fixes probably fixes #198 but since I can't repro the error, I
can't be sure.