Support for duplicate key databases (dupSort)

This release includes more comprehensive support for interacting with duplicate key databases (which are enabled with dupSort flag):

• remove/removeSync now support specifying a value as the second argument for deleting individual entries that may share a duplicate key
• getValues(key) was added to iterate over all values of a given key
• getKeys(rangeOptions) was added to easily iterate over all keys in a range, without returning duplicate keys