Move backend client timeout to configuration

The backend client timeout is coded as 3 seconds in [BackendClient.java Line 63](https://github.com/nwHacks2017/application/blob/0914c17bc93c266dd41500e018ca175923bcb606/application/app/src/main/java/com/lifeband/lifeband/service/BackendClient.java#L63). This should be moved to a configuration file so it is more easily changed.