Multiple small fixes after we have started using Jenkins-Stash integration

- Make it possible to specify the git commit sha in the configuration
We have some Jenkins jobs that do not checkout directly from git, but instead copy artifacts from other jobs based on the git commit SHA. Related to issue #7.
This code makes it possible to specify a environment variable whit the sha1 value
- Support notifying stash about in progress jobs. Fixing Issue #3 .
- Make it user configurable if repeated jobs should be shown as multiple builds in Stash. If a job failed due to outside circumstances we would like to be able to re-run it and make Stash show that the commit is green. Fixing issue #10.
- For some jobs we set the build description, we would like to transfer this to Stash.