[TIMOB-19735] iOS: Support fetching local assets

The Photos framework of iOS8 allows us to fetch local photo albums and its assets. This is useful and necessary when creating a custom photo gallery or using local photos without the modal dialog provided by Ti.Media.openPhotoGallery(). To make this possible, we have to think about, what is the easiest way to fetch and organize the photos. We will need at least a new method like Ti.Media.fetchPhotoGallery() to fetch it. After that, we could create a proxy for albums e.g. Ti.Media.PhotoAlbum and Ti.Media.Photo to access the native properties like mediaType, mediaSubtypes, sourceType, pixelWidth, pixelHeight, creationDate, modificationDate, location, duration, favorite and hidden. This assets can be photos, live photos, videos, time-lapse videos and slo-mo videos. We would also need to make sure to achieve platform parity here, since it is a very common use case on all platforms. **Related documents:** - https://developer.apple.com/library/prerelease/ios/documentation/Photos/Reference/PHAsset_Class/index.html - https://developer.apple.com/library/prerelease/ios/documentation/Photos/Reference/PHAssetCollection_Class/index.html