Organize README and add mission statement.

I added a mission statement, combined it with the outcomes section, and reorganized that section.

I also made various changes to other parts of the README to reflect our recent discussions; most of these are minor. Also hard wrapped everything to 80 columns.

Thoughts?

Possibly relevant are [this recent StackOverflow post](http://stats.stackexchange.com/questions/104500/skills-hard-to-find-in-machine-learners) and [one of Hadley's grading rubrics](http://stat405.had.co.nz/homework/rubric.pdf). I really like the ideas in the latter.
We may as well mention Vim and emacs here. It was a big revelation for me when I realized I could just take the time to learn one text editor well and use it to quickly edit and explore any file, whether code or data, in any environment including SSH. Now I use Bash + tmux + Ipython (or R) + Vim for everything. I'll probably demo this in class.

Maybe we could say something like:

Vim and emacs are efficient text editors that can be used in the terminal, but they take more time to learn. For more details see Norm Matloff's [Notes on Text Editors](http://heather.cs.ucdavis.edu/~matloff/ProgEdit/ProgEdit.html).