Null Check

Added Null/undefined check for eventName and callback params of on and
once functions.
Completed null/undefined tests

Completed null/undefined tests for ‘on’ and ‘once’ methods.
Added missing semi colon
master: moved null check logic to _addListener function from on and once methods. Added more checks around eventName args to ensure it can only be a non empty object.
master: added one more test case on() and Once() tests.
Master: Removed param check tests from on and once methods to events._addListener method.
Master: removed !eventnameand !callback checks.
updated to version 0.1.2

Updated unit tests and version to 0.1.2. Also replaced all instance of
== with ===
master: addEventListener now throws error for invalid input params.
Merge pull request #6 from invalidred/master

Fixed Need to make checks that the callback is a proper function and eventName is a proper value as well #2