Do not add functions to Array and Event prototypes
Never add matchesSelector to Element prototype
Fix use of Array#idnexOf in demo
Modify `isElement` to work on a string in IE8

https://github.com/taye/interact.js/issues/45#issuecomment-49890918
Merge branch 'dont-modify-native-prototypes'

Closes https://github.com/taye/interact.js/issues/45