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.
Instead of relying on a side-effect, can we return a value from the function to be used in an explicit assignment, like:
```javascript
var loadingTimeout = getLoadingTimeout();
```

Side-effects from a closure often make the code's intent less obvious. Especially since the function is named _get_ LoadingTimeout, I wouldn't expect it to also _set_ the LoadingTimeout.