Fix some errors when vim is compiled with +python3.

When vim is compiled with `-python` and `+python3`, jedi-vim generates
errors because of two points in the code which are not compatible with
Python 3 (because of the `str`/`unicode` vs. `bytes`/`str` thing).
Merge pull request #135 from mlmoses/fix-python3-errors

Fix errors when vim is compiled with +python3.