Add ability to insert comment from file.

Currently BitBucket pull request builder allows add only link to Jenkins job. I've added implementation which allows you specify file as source for new comment.

![image](https://cloud.githubusercontent.com/assets/1316234/5154166/20b13d80-7254-11e4-8d11-8db39a6acfd6.png)

In the Jenkins job you can write any data to file and this will be in comment on BitBucket.
```
phpcs --standard=Zend ./includes/path.inc > ${WORKSPACE}/commen.md
```