CB-2530 and CB-2239 Multipart plugin result

* Support MultiPart plugin result
* Update Echo to test MultiPart messages
[CB-2220] Fix splashscreen origin when status bar is present
[CB-2544] Document "onload" attribute of <plugin> element in Plugin Upgrade Guide
Merge branch 'refs/heads/next'
Updated Release Notes for 2.5.0
Merge branch 'refs/heads/next'
[CB-2556] Update JavaScript for iOS (2.5.0)
Merge branch 'refs/heads/next'
[CB-2220] Size the splash screen in the same way as the launch image

Also fixes [CB-2589] (activity indicator not showing).
CB-2527: Update iPad splash images to correct sizes

-Updated the following images to avoid shifting down of splash during startup:
 Default-Landscape~ipad.png   (1024x748 pixels)
 Default-Portrait~ipad.png    (768x1004 pixels)
-Reference document:
 http://developer.apple.com/library/ios/#documentation/UserExperience/Conceptual/MobileHIG/IconsImages/IconsImages.html
[CB-861] Header support for FileTransfer download

Added support for an optional options object as the final arg. Currently only handles the options.headers object (as per the issue).

`FileTransfer.download(source, target, successCallback, errorCallback, trustAllHosts, options)`

This is needed for using FileTransfer.download with Basic Authentication, etc. Sadly since Android 2.x doesn't support XHR2, this is needed in FileTransfer.

I have only added support to Android and iOS (see other PR's).
[CB-2389] Fix page load detection for late-loaded iframes
Remove NSLogs accidentally checked in.
[CB-2220] Fix splash screen positioning when image is the size of device