SQLite database cross-platform extension implementation.

This is a continuation of a previous pull request [closed] that implemented cross-platform SQLite API directly into the Libgdx core. Ref: https://github.com/libgdx/libgdx/pull/222

As per the small discussion, I have implemented it as a separate extension that currently supports Desktop and Android.

The Desktop implementation uses SQLite JDBC from https://bitbucket.org/xerial/sqlite-jdbc/wiki/Home while the Android implementation only uses the Android SQLite API.