Implement EagerThenables(). Separate thenable, eager for inclusion via browserify

• Implement nodent.EagerThenable() to provide Promise-like (but unchainable) execution semantics (eager evaluation, asynchronous resolution)
• Implement new test harness to collate performance by mode and Promise implementation
• Allow optional passing of a Promise type to the covers http, https, map, events and movre Thenable to it's own fix to ease integration with Browserify or Webpack (specifically, these covers can be required directly as there is no hard dependancy on the 'nodent' parameter, and so no need to require the entire library into thebrowser). The default behaviour is now to use the global.Promise if present, or nodent.Thenable if not.