Open CDTDatastore's underlying TD_Database on -init

This helps to make sure initialisation code is only called
from one thread.
Use local CouchDB instance for all attachment replication tests
Send the right data in -addAttachmentToRemoteDocumentWithId:...
Show we don't support replicated gzipped data yet

Test which replicates gzipped data from remote database. We
need to fix exposing this data through our attachments API.
Use NSData rather than NSInputStream in attachments API

The iOS and OSX APIs appear to generally prefer working with either
NSData (e.g., UIImage imageWithData:) or file paths/URLs (e.g.,
UIImage imageWithContentsOfFile:). So providing NSInputStream
interfaces, while potentially more efficient, seemed like it would
be an overhead -- people would be forever writing these to files or
NSData objects anyway.
Allow running the replication acceptance tests from rake
Inflate gzipped attachments before returning data to client

Some content types are zipped before storing on disk during
replication (or they come compressed from CouchDB and saved
to disk without removing the compression).

This also paves the way for us to compress certain types when
added through the Cloudant Sync API.