Add build log link to attachments

I found myself needing a link to the build log as in [this issue](https://github.com/Tapadoo/TCSlackNotifierPlugin/issues/7).

I added the build number with a link to the build log to the top of the attachments. I also moved the build duration into the attachments to make the message easier to process visually - I hope that's not too presumptuous. This is the output for a build (with no changes):

![image](https://cloud.githubusercontent.com/assets/1592077/3422350/b7810b5a-ff31-11e3-83cd-b82f0fb13137.png)

Thanks for building this! It's incredibly helpful.