Issue #595 - updated the documentation and example to reference isLabel.

Updated the documentation for <markers> directive to reference the new isLabel property. Also updated the example code to show how to use this property.
Yes it is supported, if you could change this to {expression}

https://github.com/angular-ui/angular-google-maps/blob/master/example/issue-74-markers-events.html#L48
https://github.com/angular-ui/angular-google-maps/blob/master/example/assets/scripts/controllers/issue-74-markers-events.js#L84-L91