Support for Aborting Transactions

[hiranya911]

#54 implements transaction support where the ref.transaction() method accepts an update_function(). Following changes are proposed to this API:

• update_function() may signal abort by raising an exception. This is probably how this API works today. Verify and fix if needed.
• transaction() method returns the updated value when it commits. If retry attempts are exhausted it should raise an error.

[hiranya911]

These changes are now implemented.