couchdb: use generic error handling also for HEAD requests
couchdb: responseRev should actually return errors

this regression was introduced in 9a4309c
couchdb: improve documentation
couchdb: add Rev method
couchdb: add PutAttachment
couchapp: rewrite

- it's not just for design documents
- use glob patterns instead of regular expressions for ignore list
- support for documents loaded from a single JSON file
- support for attachments