EventEmitter#eventNames() and Symbol support

[lpinca]

This addresses #6 and add support for symbols.

While I was writing this, I found an issue (see inline comments) that should be fixed, so don't merge it yet.

[lpinca]

It is possible that we are in a browser that doesn't have Object.create support and this is an instance of EventEmitter3 < 1.2.0.
In this case we fill the array with prefixed event names, and this will obviously fail below when we get the listeners via ee.listeners(name) as our name will be prefixed again.
The ideal solution would be to detect if this is an instance of EventEmitter3 and in that case see if it is prefixed and slice the event name accordingly.

The problem here is that in order to do this cleanly we need to add a new method to EventEmitter3 and publish it as 1.1.2. Another option would be something like this:

var EE3;
try {
  EE3 = require('eventemitter3');
} catch(e) {};

and check if ee is an instance of EE3 but this seems fragile.

Do you have better ideas?

[lpinca]

It seems that there aren't modules for the browser that depend on ultron (I'm not sure about truth and fittings), so we can also assume that we are safe if we end in that branch as new installation of eventemitter3 will use eventNames and we only end there if we are using node.

[lpinca]

@3rd-Eden I would go like this. People affected by the bug described here, if any, can upgrade eventemitter3 and ultron to the latest versions to have it fixed.