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.
How about 'easier to read, write, test, and reuse'. Might be good to sneak the word 'modular' in there as well.