Put resources into a bundle

## Motivation
This MR makes all resources (images and strings) available in a bundle (CTAssetsPickerController.bundle) as opposed to being copied individually into the main bundle. This aims to clean the integration of this pod within the projects by containing all resources inside a bundle instead of scattering them all over the root of the App.

There are three main tasks for achieving this:
 * Create the bundle with the resources inside
 * Make the code load resources from this bundle instead of the main bundle
 * Adapt the demo project to this new reality

### Creating the bundle

This was the easy part. Using `resource_bundles` in pod spec, we can specify which files get inside each bundle. I just created one bundle named `CTAssetsPickerController` with all the images and localization directories inside. This bundle will be copied to the final product, instead of all the files mentioned in `resources`

### Loading resources from the bundle

To load the resources from this bundle I had to change every resource loading method `NSLocalizedStringFromTable()` and `-[UIImage imageNamed:]` to load from the CTAssetsPickerController.bundle instead of the default main bundle. I created two new methods, `CTAssetsPickerControllerLocalizedString` and `- [UIImage ctassetsPickerControllerImageNamed:]` that do this. These methods are defined in two categories of `UImage` and `NSBundle`.

### Adapting the Demo project

The Demo project was changed to use the pod as a library (much like any project using this pod) instead of having the .m files and resources directly in the App target. For that I added a Podfile for the demo project with a reference to the current directory. This will use the podspec in the current directory to build the `CTAssetsPickerController.bundle` and pod library.
I then removed all CTAssetsPickerController/* files from the demo target because these files will be compiled into the pod library that will be linked to the demo by cocoa pods. Also removed .xcassets and .strings from the demo target because they will be included in the bundle that will be copied to the final product.
I also updated the project settings to the ones recommended by Xcode and added the Pods directory to gitignore.

I know this is a big MR, if you have any doubts please ask me about it.

Thanks!