Enable syntax highlighting for code examples in readme file

I'm not sure if this is necessary or not but to make code examples a bit easier to read, I enable syntax highlighting for each code block using [Github Flavored Markdown syntax](https://help.github.com/articles/github-flavored-markdown#syntax-highlighting) without changing any content of the code.

Anyway, I followed along the code examples and found some issues (I'm using Cordova 3.0):

- `e` object in `onNotificationGCM()` is changed. `regID` is now `regid` and there are no longer `e.foreground` or `e.coldstart` available.