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
Why are you catching a `Throwable` here?  I think it's better to explicitly catch each exception that can be thrown.

I think you should have separate `catch` blocks for `ProjectManagerException` and `IOException`. It might be good to log a message, too, if there is some useful information you could provide.