Update README for shorter gem description

Hoe parses out the `## Description` section for use in the gemspec description,
which at present is very large and uses markdown that the gemspec / rubygems
doesn't care about.

This commit replaces the description with an abbreviated version, and puts the
full description below a new header "built for speed and concurrency"
Merge pull request #247 from catsby/update_gem_description

Shorten the gemspec description