Gem-ify ExceptionalSynchrony

This adds the minimal changes necessary to convert ExceptionalSynchrony
into a gem. Most code changes are just plumbing for gemspec / test
helpers, etc, although in several locations methods from Hobo Support
are removed.
Add some more missing pieces

`ruby test/unit/*.rb` is a poor way to try to run tests, derp. This adds
some pieces that got missed the first time around.

Added dependencies: em-synchrony, webmock

Certain tests are still failing due to missing constants, e.g. PNAPI and
ExceptionHandling.
Merge pull request #1 from andrewberls/master

Gem-ify ExceptionalSynchrony