Use 'com.android.application' instead of 'android' for gradle plugin.
Lower minSdkVersion to 15 instead of 19 to broaden compatibility.
Use slf4j-api and logback-android.

- Get rid of the custom logger
- slf4j-api provides a convenient set of Logging APIs which can be
  customized by the logging implementation.
- Use logback-android as the logging implementation.