Adds a gitignore file and the ability to choose a photo from your photo library

This commit adds a UIImagePickerController to the REComposeViewController which is handled directly by the controller. If a user clicks on the attachment view, the UIImagePicker will be displayed and will allow a user to change the attachment in the composer. If a user has updated the attachment the userUpdatedAttachment property will be YES, and the attachmentImageView will have been changed to reflect this choice.