Add test for attr()

Note that this currently fails as $.isObject(null) returns
true, and the test value != undefined returns false if value is
null.
Fix $.isObject() and condition in attr()

This allows $(selector).attr('attribute', null) to correctly
remove an attribute from a collection of elements.

Fixes https://github.com/01org/appframework/issues/349
Merge pull request #350 from townxelliot/attr-tests

Tests for attr() with fix for $.isObject() and fix for conditions in attr() itself