Mark data I/O methods in AlternatorDBHandler 'synchronized' (on 'this').

This is a low priority update that addresses issue 58:

  https://github.com/mboudreau/Alternator/issues/58

This addresses development in environments such as Node.js where a set of asynchronous worker thread can be simultaneously spawned to perform database I/O operations.

Since the AlternatorDBHandler is used as a singleton, the 'synchronized' keyword was simply added to appropriate methods.

  http://stackoverflow.com/questions/574240/synchronized-block-vs-synchronized-method