revamp introduction

The intro was getting a bit outdated. What do you guys think about this?
We also need a TL;DR version for the README.
This language is a bit weird, since the next two bullets are (arguably) statements about what JuMP shares with e.g. AMPL. I'd change this to "particularly attractive features of JuMP", or reorder things so that solver independence and low-level features (the really unique stuff) are up top.