Add wait flag to ResourceManager

[andyneff]

Right now, if the ResourceManger runs out of resources, it will through an exception by design. Some people would prefer it to wait instead so that we could start to have tasks that only partially use the resource and share the resource.

Perhaps adding a flag to the .get_resource method could allow for certain pieces of code to allow for this over committing of resource sharing. A timeout of sorts should probably be used to prevent an infinitely waiting. and a FIFO queue should probably be added to add FIFO behavior.