Support the ability to download project zip file #373

* A new button is displayed on the project page to download the project zip file
* One can use curl to download the project zip file
curl -O -J -k --get --data "session.id=<session id>&project=<project nane>&download=true" http://<websever host>:<port>/manager
Does this mean Azkaban keeps around every version of a project that's ever been uploaded? If I upload a 100 MB zip 100 times, does that mean Azkaban will store 100 copies of my zip in the DB?