Update Mapbox Android SDK

This is now using version `0.2.3` of the Mapbox Android SDK. Along the way, I did a number of things:

* A new `HttpManager` class has been added, with an `init()` method that can be called idempotently, which installs `OkHttp` as the backing HTTP workhorse for `HttpUrlConnection`. This is necessary, as `OkHttp` and plain `HttpUrlConnection` [cannot play nice together](https://github.com/square/okhttp/issues/673#issuecomment-39207589).
* The Google News and MoC photo fetching classes were refactored to use `HttpUrlConnection`, and not the deprecated ([in Android](http://android-developers.blogspot.com/2011/09/androids-http-clients.html)) `HttpClient`.
* The project now uses Gradle for building and dependency management. Some `.jar` files are still embedded in the project, such as Google Analytics. `build.gradle` loads those. From a developer perspective, I am now using Android Studio `0.5.2`, and not Eclipse.
* The Mapbox Android SDK has been updated, and uses a built-from-source copy of `0.2.3-SNAPSHOT` as a `.aar` file, in `aars/`. This version of the SDK also contains a modification by me, that fixes up a crash in GeoJSON downloading and parsing, that is currently an outstanding pull request over in https://github.com/mapbox/mapbox-android-sdk/pull/328.
* However, we're not using Mapbox's GeoJSON parsing, because it [doesn't yet parse MultiPolygons](https://github.com/mapbox/mapbox-android-sdk/issues/205#issuecomment-40667790). We're using a homegrown district parser, but now using Mapbox's direct rendering library, not the old `osmdroid` rendering library. (The differences in syntax and display are not large.)
* Hardware acceleration has been disabled, as the Mapbox team has identified [trouble with PathOverlays](https://github.com/mapbox/mapbox-android-sdk/issues/320), which the app uses to draw to the map.

But most importantly:

* The **minimum supported Android version is now 2.3.** Google Analytics suggests 2.2 is an extremely small percentage of the user base (< 1%), so we can kill it and simplify the code and views around map loading. The Google Play publisher statistics suggest the % of "current installs" of 2.2 is higher - around 4%. But active users is more important.

Google Analytics also says 2.3 represents just 3% of the user base, but given that Google Play says it's 20% of the install base, and there's no obvious short term win in cutting off 2.3 support, no reason to yank support yet.