#3 - (1) Logging to external file; (2) Gradle config

Related to the issue #3

1. When in release mode, logs are written to a file on external
storage.

New EnvironmentHelper class determines what mode application is running
in. Loggers are now obtained using LogManager, not instantiated
directly in classes.

Also refactored ILog interface and ConsoleLog, FileLog classes to
include logging of messages with format and other changes.

2. Setup Gradle release configuration. Key store and key information
are stored in environment variables.