Support MeshTypes in JobContent

[robertmaynard]

As described in #5 we need to add mesh types to Remus.

It is pointless to add mesh types to Remus unless we can use them inside a job's content. This requires an extension of the remus::common::ContentFormat types and more constructors for the JobContent class.

Lets say that we add a ContentFormat type of REMUS_MESH, we would need a way to give a remus::mesh::Mesh to a JobContent, so we would need to add the following constructor:

JobContent(remus::mesh::Mesh m);

This constructor would create a JobContent with a ContentFormat of REMUS_MESH, and a SourceFormat of MEMORY

Now the other challenge is getting the JobContent to easily convert its contents back to remus::mesh::Mesh.