Supply loading_timeout attribute for jsinput tag.

This is an attempt to address a request from MITx regarding the loading timeout in JSInput. The default value of 300 ms doesn't work with these very heavy GWT TEALSim applications in 8.01r. This PR lets an author choose this timeout from the XML, as an attribute of the jsinput tag. If multiple JSInput have this attribute set and are included in a page, the largest value will be retained. The only issue I know of is that pressing on the Check button before the timeout has elapsed will result in a JS error thrown in ```check_save_waitfor``` with the corresponding alert: ```Could not grade your answer. The submission was aborted.```. This is known and deemed acceptable by the developer that issued the request.

@carsongee @pdpinch @terman  @stvstnfrd @jinpa @Lyla-Fischer Please have a look.
@stvstnfrd 
> Why is loadingTimeout applied globally, instead of being applied to
the element on which it was declared?

Good question. The developer integrating the TEALSim applications for MITx reported he would get the following exception on page load: ```Error: could not set state: Timeout```. He tried modifying the timeout in the while loop (line181) or increase the number of loops (line 188) with no success. Only by setting 300 to 3000 would he get his setstate function to work. The page wouldn't freeze, only issue, the check button wouldn't work for 3s. That's all I got  as I do not have access to the code integrating these applications and am only replying to a request. I will email the developer to let him perhaps add additional information.