Added attachment

Added the much-needed attachment configuration option. Now you can
attach something to a tweenable instance (e.g.: a DOM element, an
object, a string, etc.) and it will be passed to start/step/finish
methods as a second parameter. Demo available.
Typo in demo file
Updated versions, comments, added new demo

I have updated the versions to SemVer spec and added a new, more
polished and performance optimized demo file "attachment-demo2.html".
Removed null enforcer for undefined attachment.
Typo fixed.

"First" typo fixed.
Merge pull request #43 from adrianvoica/master

Added "attachment" functionality to setConfig.