Added possibility to set custom bundle where core WebViewJavascriptBridge.js script is located.

I added new field and new initialization method to be able to set custom NSBundle where `WebViewJavascriptBridge.js.txt` file is located. 

I'm developing a static library which uses `WebViewJavascriptBride` and I need to export all resources somehow. With CocoaPods it is easy but if I want to build an `.a` library file I need to export all resources to `Resources.bundle` and place it next to `.h` header files. It would be nice if I could just pass additional argument with this bundle so that `WebViewJavascriptBridge` can find it's dependencies there :)

If you find it helpful feel free to merge it to your repo. I tried not to make any needless changes.