Upgrade plugin to parent version 1.424

Implemented the new SCM methods, including the new method of polling for changes.

Also refactored a bit of code to reduce duplication.

Manual testing:
1. Ran `mvn` and copied the resulting `target/tfs.hpi` to a Jenkins server running version 1.424, restarting it to make sure my version was picked up. (the Jenkins server is running under Windows)
2. Double-checked same by navigating to `http://jenkins.example.com/pluginManager/installed` and confirming that the version listed was *1.21-SNAPSHOT (private-06/24/2013 20:52-odagenais)*.
3. Created a job that uses the TFS SCM, forced it to execute on a Linux agent (which has TEE CLC installed) and manually triggered its build.  Success.  Recorded changeset number displayed in log (let's call it `42`).
4. Configured that job's polling to every minute and saved it.  Refreshed `http://jenkins.example.com/job/<jobName>/scmPollLog/` until I saw that it ran.  The log contains `-version:C42~D2013-06-18T19:43:40Z` which confirms the new polling method is being run.  Awesome.
5. Edit the job and changed its `Project path` to an entirely different path and saved the job again.  Refreshed the Polling Log page until the "Started on" time was different.  When it was different, this time the log didn't even include a call to `tf history`, it completed in 6 ms with "Changes found".  Double awesome.  A build was automatically triggered, whose console output contained the line `Deleting workspace as the configuration has changed since a build was performed on this computer.`.  Recorded changeset number displayed in log (let's call it `27`).
6. Visited the Polling Log page one last time, confirming that it again was using the new method and that there were no changes: `-version:C27~D2013-06-18T19:50:40Z`
7. Checked that jobs created before the TFS plugin was upgraded gracefully degraded to the old method of polling after upgrading the plugin and that the upgrade did not needlessly cause new builds to be triggered.  Triggered builds both by SCM change and manually and confirmed that in both cases, they switched to the new polling method afterwards.
Mission accomplished!