Implement endpoint for HEAD, GET and PUT /dbname/ requests

Note, that requests comes for /dbname/ endpoint *with* trailing slash.
As for storage, there will be an abstract database class which will
be extended by project needs.
Implement basic in-memory storage with update conflicts model
Implement basic document API
Implement /db/_revs_diff endpoint
Implement /db/_bulk_docs endpoint
Implement /db/_ensure_full_commit endpoint
Use pickle instead of json for revision generation

Removes problems with unicode/bytes content for attachments.
Support replication of documents with attachments via multipart API