fix behaviour of Gauge: it used to behave like GaugeDelta when passed…

… a positive value

* Added stdout client ("echo" service for debugging)

* Fixed issue #23: GaugeDelta event Stats() should not send an absolute value of 0
* Fixed FGauge's collation in the buffered client to only preserve the last value in the batch (it mistakenly had the same implementation of FGaugeDelta's collation)
* Fixed FGaugeDelta with negative value not to send a 0 value first (it mistakenly had the same implementation of FGauge)
* Added many tests
* Added compile-time checks that the default events implement the Event interface

updated changelog

new minor version: Added SendEvents function to Statsd interface; Usi…

…ng interface in buffered client constructor; Fixed tests

Merge pull request #37 from treaster/master

Fix test failures introduced by a65ea2a