Allow external application to retrieve the current Artwork details and Bitmap via a ContentProvider

Instead of external applications needing to re-implement all of Muzei or redownload images that Muzei has already cached, we provide a ContentProvider with a single table and single row which stores the current Artwork details and implements openFile() to give access to the already cached artwork Bitmap. MuzeiContract has been added to the api library as well as helper methods for serializing/deserializing Artwork with ContentProvider classes.
Adds example-contract-widget to show usage of Muzei ContentProvider

This example implements a simple full screen widget such as that requested in #58 as a separate example app by utilizing the Content Provider and ACTION_ARTWORK_CHANGED broadcast to get callbacks on when the artwork changes and read the information without the need for any permissions (such as INTERNET to download the artwork).
Bridge the Muzei Content Provider onto Android Wear devices

Even more important for Wearable apps wanting to use Muzei, this ensures that artwork only needs to be transferred to the Wearable device once and other apps can locally retrieve the current artwork details and image rather than have multiple apps send the same images through the Wearable Data Layer.
Fix Data Layer asset key and edge case in Content Provider

Mistyped Data Layer asset key, causing images to be ignored on the Wearable side. Also fixes an edge case where no artwork location was set, causing a crash due to use of new File(null).
Ensure wearable is signed with the same keys as main

Wearable will not install for release builds unless it is signed with the same key as the main application. This also enables a separate version.properties file for wearable and adds proper proguard support.
Bump api version to 2.0, minor code cleanup
Rename net.nurik.roman.muzei constants to com.google.android.apps.muzei

Make note of :api being transitively included in :main due to dependency on :provider
Merge pull request #94 from ianhanniballake/provider

New API: Retrieve current Artwork details and Image