Artifact Store Feature

Following functionality should be implemented:

## Artifact Store

Marathon needs to know where to store the artifacts.
The url to the aritfact store is given as command line parameter on start.
Example: --artifact_store hdfs://my.node/directory/to/store

## Artifact REST endpoint

The artifact store can be accessed and managed via http.
GET /v2/artifacts/{path} Download artifact with given path.
POST /v2/artifacts/{path} Upload artifact with given path in the artifact store. Return the complete URL of the artifact in the artifact store in the location header.
POST /v2/artifacts Upload artifact. A name is generated by the server. Return the complete URL of the artifact in the artifact store in the location header.
DELETE /v2/artifacts/{path} Delete artifact with given path.

## Automatic Artifact Resolution

Marathon can use the artifact store to automatically resolve certain artifacts.
The AppDefinition is extended with Parameter resolveUrls: []
Those urls get 
* resolved 
* stored in the artifact store
* the artifact store urls are added to uris property
as part of the deployment.
Fair enough.

For what it's worth, Rails supports POST for DELETE and PUT but enables overriding the method with the `_method` parameter. If your client doesn't support the appropriate REST verb (like plain HTML forms), add a `_method=put` parameter to fake it. That could be an abstraction handled elsewhere: when you define an action as PUT it enables the verb overriding with `_method`.