DOC: Lightly edited README

This is mostly rewrapping text, using "examples" instead of "tutorials," and fixing a broken link to `numba`.

There are 2 possible substantive changes:

- I changed the advanced install instructions to suggest `pip install -e` instead of `python setup.py develop`. Seems like good practice, and more consistent.
- I changed the novice instructions to install `numba` 0.12.2 instead of 0.11.0. In my judgement, the 0.12.x series actually replaces 0.11 now.