Exposed TFSRevisionState to the API

It is now possible to perform a GET like the following:
`http://server/job/myJob/17/api/xml?tree=actions[changesetVersion]`
...which will return XML like the following:
```
<freeStyleBuild>
    <action />
    <action />
    <action>
        <changesetVersion>44392</changesetVersion>
    </action>
    <action />
</freeStyleBuild>
```
This makes it possible to retrieve the TFS changeset that was used for the build, even if the build wasn't triggered by a TFS changeset, such as being queued manually.