added getCaption() and setCaption() methods to titlebars on BBOS5,6,7 devices.

added getCaption() and setCaption() methods to titlebars on BBOS5,6,7 Devices. Usage is now 

```javascript

var tb = document.getElementById('myApp_titleBar');

// when no argument is passed in, titleBar text is taken from the 'data-bb-caption' attribute; useful when other code has modified the attribute indirectly
tb.setCaption();


// set the titleBar text and the 'data-bb-caption' attribute to the value of myNewText; useful when other code needs to modify the titleBar directly
var myNewText = "Updated titleBar text";
tb.setCaption(myNewText);

alert(tb.getCaption()); // alert dialogue with "Updated titleBar text"

```