Publish OkHttp extension to Bintray.

It'll be a TODO to clean up some of the commonality between
publishing this and the core library (e.g. the version number
should really be spec'd in one place only). Just getting this
submitted for now so that we don't lose track of how I created
https://bintray.com/google/exoplayer/extension-okhttp

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135102459

extensions/okhttp/build.gradle

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 apply plugin: 'com.android.library'
+apply plugin: 'bintray-release'
 
 android {
     compileSdkVersion project.ext.compileSdkVersion
@@ -40,3 +41,13 @@ dependencies {
         exclude group: 'org.json'
     }
 }
+
+publish {
+    repoName = 'exoplayer'
+    userOrg = 'google'
+    groupId = 'com.google.android.exoplayer'
+    artifactId = 'extension-okhttp'
+    version = 'r2.0.1'
+    description = 'An OkHttp extension for ExoPlayer.'
+    website = 'https://github.com/google/ExoPlayer'
+}