Add Bower support.
Ensure browser build is basically okay.

Doesn't run extensive unit tests, but ensures basic promise
functionality is there and that things don't error out. The Mocha tests
in the Node environment should be sufficient for testing functionality;
as long as the packaging is set up correctly everything should work
exactly the same.

NOTE: holdup should be ES4-compatible, but the test page uses some
HTML5-isms that might break in older browsers.