start on a generic adapter class and an in-memory storage adapter
use module pattern in generic adapter
bulkDocs using generic backend
generic getAttachment and allDocs implementations
mem adapter: keep track of update_seq, and don't hand out internal objects
extract and rename IdbPouch.Changes to Pouch.changes
generic adapter: changes
add adapter.type() method, make sure callback gets called after changes() and allDocs()
add mem adapter to tests
fix typos and add mem to adapters being tested
return changes starting at sequence immediately *after* 'since' parameter
call onChange after de-duping
make generic adapter code browser-friendly

- remove dependency on crypto module
- only set module.exports=PouchAdapter in node
- set Pouch.utils in browser
add mem adapter to browser tests
make mem adapter API async, fix atob/btoa 'Illegal Invocation' error