implemented .on(), .off(), .once()
added events to available methods section in readme
use event.target instead of event.currentTarget
added comments for event code
fixed broken tests
use es6 WeakMaps for event references
added polyfill for WeakMap
updated .off() method to make it more readable
added missing tests and fixed code for using .off() without handler function
refactored .on() method for readability
Merge pull request #33 from garthenweb/on-off-once

implemented .on(), .off(), .once()