Make _session endpoint accept username in addition to name
Make _session endpoint accept username in addition to name

Before _session endpoint was only accepting "name" and "password" parameters
This, makes _session endpoint, in addition,  to accept "username" and
"password" parameters

JIRA: COUCHDB-2754
BugzId: 47059

cloudant/couchdb-couch,KlausTrainer/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,KlausTrainer/couchdb-couch,smithsz/couchdb-couch,KlausTrainer/couchdb-couch,fkaempfer/couchdb,ngpestelos/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch,smithsz/couchdb-couch,ngpestelos/couchdb-couch,fkaempfer/couchdb,ngpestelos/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch,cloudant/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch,smithsz/couchdb-couch