Always use webkitRequestFileSystem to prevent collisions with the Apache File Plugin.

As pointed out in Issue #320, if people are using the Apache File Plugin, the `readFile` function throws calls the `errorHandler` with `error.code = 1`. This is because the Apache File Plugin overwrites `requestFileSystem`.

Since there's no reason to use `requestFileSystem` over `webkitRequestFileSystem` on BB10, this pull request removes the dependancy of `requestFileSystem` in favour of always using `webkitRequestFileSystem`.