#300 rev id unique per doc - websql - indexeddb

Make rev ids unique per document. Not sure how to implement tests. Basically would mean you create two empty docs in couchdb ( both revs should be "1-967a00dff5e02add41819138abb3284d" ), and see if both get stored after replication.

Had a look at chesles' implementation of leveldb to see if we could do it that way, but I prefer the idea of using the native indexes instead of creating a map of our own for each doc.