Need to look at e.srcElement usage

[toannguyen83]

In some condition, firefox doesn't set the e.srcElement. See #24 for example.
There are other places that we're using e.srcElement that we need to look at.

[ryoe]

We'll just make sure we try e.target, then e.srcElement everywhere... note the quirksmode link below for Safari-specific issue.

References:

• http://stackoverflow.com/questions/13602039/e-srcelement-is-undefined-in-firefox
• http://www.quirksmode.org/js/events_properties.html