SBML support in API

[devkhan]

Right now, the API only support SBML <--> Escher conversion. The command line executable does support SBML export, but this is not implemented in the API (and hence, web app).

The important decision to make here is whether to allow layouts of an SBML to be downloaded (like /output/{file_id}/{layout_id}) or just the first one.

This needs to be done before #27.