Improve $PYTHONSTARTUP support

This patch introduces two main changes:

- Add minimal document for $PYTHONSTARTUP
- Try rlcompleter first then fallback to jedi.Interpreter

Discussion:

- Shoud `jedi.utils.*` be importable at top-level?
  In other words, which one should user use: `from jedi import setup_readline` or `from jedi.utils import setup_readline`?