getPropertyValue validation

Prevent access from getPropertyValue to functions or other possible CSSStyleDeclaration properties
Validate item() method of CSSStyleDeclaration

Ensure CSSStyleDeclaration is converted to an integer (and thus prevented from accessing functions or other properties). (At least Mozilla does this conversion.)
Merge pull request #1 from brettz9/master

Validate item() method of CSSStyleDeclaration
Throw error if argument is undefined to item() or getPropertyValue()
Merge pull request #16 from brettz9/patch-1

getPropertyValue validation