Use Blobs instead of Base64 for attachments (http)

Note that I distinguish between inline and standalone attachments,
like Couch does.
With inline attachments you still have to use base64,
whereas putAttachment as well as get('docid/attachment') use blobs.
reactivate accidently checked in disabled local adapter for attchment test
update docs to reflect attachments blob usage
adapt pouch.adapter blob storage
Implement Blob attachment usage for IndexedDB

Note that this will need further improvement.
Above all the calculation of the md5 digest needs to be more efficient,
currently (as before) the whole attachment is loaded into memory.
adapt pouch.adapter to reflect new putAttachment signature