Migrate to pytest

This PR completes migration to py.test I started at #153.

Some remaining works.  Tell me if they are needed to be done here:

- Remove old test code.  Currently you can still use old runner.
- Get rid of `XDG_CACHE_HOME={envtmpdir}/cache` in tox.ini.  You can monkey patch `jedi.settings` now.
- Document how to run py.test.

BTW, .travis.yml can be simpler like this if you are OK with loosing nice matrix view in travis.org:
```yaml
language: python
install:
    - pip install --use-mirrors tox
script:
    - tox
```