Allow artifacts to be passed from Server to Agent

The bamboo clover plugin, needs access to all of clovers history points before generating a clover history report.
If the build is run on a different agent than it was previously, then not all Clover history points will be available.

The plugin needs a way to:

• access a directory of files, that were generated by previous builds.
• specify that an artifact must be made available to subsequent builds.

One way, although not ideal, would be to store such artifacts in a .bamboo directory on the agent, in the plan's build-dir.
The agent would then retrieve any PLAN level artifacts and store them there, prior to starting the build.

• Artifacts could be retrieved via HTTP, from the bamboo server.
• At the end of the build, the .bamboo directory is zipped, and sent back to the server and stored as a PLAN level artifact.
• The location of the .bamboo directory would need to be made available to any build plugins via a property, so that it can be passed to the actual build.

I believe this would also be useful for passing other artifacts between builds, such as an application.war from one build, to other dependent builds that then test that war.