android: add phonegap 3.1 tweak

Start from phonegap 3.1,
- DroidGap class is no longer used. It's replaced by `CordovaActivity`.
- `init()` method is called explicitly in the `onCreate()` method. To 
make Moodstocks phonegap plugin work properly, this line need to be 
replaced by `this.init()`.

A short description is added too.
android: set web view background to transparent

This solves the random launch flickering problem.
iOS: Fix memory leak & regression in issue #6

This also fix the regression caused by the commit b8c6f32acf that closes 
issue #6. The regression is showed as `sync` method no longer receive
the callback messages.

The solution involves using CFRetain & CFRelease to release the plugin's `self` object. It's also important to set syncHandler & scanHandler as
`nil` otherwise it will cause a memory leak.