Simplify URL management in configuration

[lfoppiano]

Instead of having to deal with port and server name I've modified the configuration to allow a single string that gives the user the flexibility to configure http/https, port and eventually application paths

The configuration now requires just one entry like:

"grobid_server": "http://localhost:8070"

or

"grobid_server": "http://myserver.com/path1/path2"

The client will add the various path e.g. /api/processXYZ as required