add job.get_build_metadata

For large builds, this method can be significantly faster!
add comment about depth control
Merge pull request #285 from jduan/get_build_metadata

Get build metadata
add get_causes
make tests more robust
Merge pull request #286 from jduan/master

Add build.get_causes
Example of adding a shell build
Merge pull request #288 from timp21337/master

Example of adding a shell build