Add new interface and method for sending command

Sending command job is giving up if command cannot be sent with any
reason. It means that command would be lost if gerrit is down.

This patch adds new interface then implement it to abstract class
to raise exception if command was not sent with errors.
User can implement retry logic using this.
Merge pull request #22 from rinrinne/send-command

Add new interface and method for sending command
Added dates to Change and Patchset

Added createdOn for Change and Patchset as well as lastUpdated
for Change. These values were previously thrown away but are now
saved.

Change-Id: I1fef9a7019101e89afb942d640168a43ab2b15ba
Merge branch 'i' of gerrit-events
Update CHANGELOG.md
[maven-release-plugin] prepare release gerrit-events-2.3.0
[maven-release-plugin] prepare for next development iteration