Feature: Add support for promises

[lsquires]

Added support for promises and async/await to callback functions. Added examples and explanation to readme.
The api callback functions now have a promise based variant with the same function name appended by "Async".

[thisredone]

This is basically what you can accomplish already with the bluebird's promisifyAll function. A proper promise/async support should work by always returning a promise and letting the user omit the callback.

[jkoudys]

I'll echo @thisredone 's comment, and also point out that node8 even includes promisify as part of the builtin utils now. We should be making proper async code, not merely wrapping error-first callbacks.

What concerns me is that this PR is sitting open, when it should have been rejected and closed months ago.

[chrisdukakis]

Thank you for the PR! It turned out that a rewrite was required to properly integrate promises. Promise support will be introduced in v.1.0.0 and is currently implemented on next branch.