Use context / represents / alias in CX assembler to represent grounding

[bgyori]

See http://www.home.ndexbio.org/data-model/

• Use context to abbreviate identifiers namespaces

"context":[{
       "ncbigene" : "http://identifiers.org/ncbigene/",
       "kegg.pathway" : "http://identifiers.org/kegg.pathway/"
     }]}

• Use nodes / "r" attribute to represent primary grounding

{"@id" : 7,
"n" : "Pentose phosphate pathway",
"r" : "kegg.pathway:hsa00030"
}

• Use nodeAttributes / "alias" to represent alternative grounding

[bgyori]

After some review, I determined that this should be done after #70 and #238.