Update Gradle configs
Add task to upload to Sonatype

* Adds following tasks: jarRelease, jarDebug, uploadArchives
Add Travis-CI
Prepare for next development iteration
Merge pull request #24 from BoltsFramework/grantland.travis

Add Travis-CI
Merge pull request #22 from BoltsFramework/grantland.sonatype

Add task to upload to Sonatype/Maven Central
Use AndroidExecutors on Android

* Fallback to java.util.concurrent.Executors on JVM
* Publicize bolts.android.AndroidExecutors
* Create BoltsExecutors