(#1132) - ensure that destroy() deletes docs

Previously, destroy() would not actually delete data,
and you could recover it by recreating the database,
due to mistaken prepending of '_pouch_' to the database
name.
(#1137) - Harmonize db creation via opts/string

Ensures that destroy() and the PouchDB() constructor work
the same, regardless of whether the database name is
specified via options or a string.