Working Tests

This configures the project to make the tests run again. Several changes are included.

### Test plugin
Rather than using the deprecated Novoda plugin, the plugin distributed by the Robolectric team is used. There are two big changes here. The first is that the `robolectric` task has been replaced with the `test` task. The second is that the source files can be put in `src/androidTest/java` directory. This is good because the IDE then picks them up without needing the `addTests` task in the build file.

### Test Files
Robolectric does not work with version 19 or 20 of Android. To work around this, all the tests have been marked as targeting Android 18.

### Build File Updates
- Version numbers have been fully defined. This eliminates all the lint and compiler warnings.
- The demo project references the local library rather than a SNAPSHOT which is technically a development version behind. This means the sonatype snapshot repo no longer needs to be included.
- The new robolectric gradle plugin is on maven central, so maven local is no longer needed.
- Android tool versions bumped to latest.

### Travis CI
A configuration has been added for Travis CI to run the tests. You just have to go enable it at https://travis-ci.org/
Moved here because these plugins are only used by the library