Ensure MultiChangesFeed goroutine stops when continuous feed closes

Otherwise we leak goroutines. (Applies to continuous and websocket feeds)
Changes feed now allows 'since' to be specified in string form

If the "since" parameter in a JSON POST body is a numeric string,
convert it to a number instead of starting over from sequence 1.
Revert "Changes feed now allows 'since' to be specified in string form"

This reverts commit 206cb2422b58a5d32ac16e545b41358cc0b36696.
Changes feed now allows 'since' to be specified in string form

If the "since" parameter in a JSON POST body is a numeric string,
convert it to a number instead of starting over from sequence 1.
Fix shadower creating spurious duplicate revisions

Problem occurred when docs got replicated into the db and then copied to
the shadowing bucket, and the shadower saw the echo from that bucket and
made a new revision out of it instead of realizing it was an echo.
Fixes #296
Update README.md

Added more links. People keep asking "how do I build it?"
Handle stub attachments better when sending multipart doc responses

For couchbase/couchbase-lite-ios#283
Rewrite _attachment dicts into valid form when adding doc to db

Make sure we don't end up with weird stuff in one, that could cause
trouble later on.
For couchbase/couchbase-lite-ios#283
"channels" param for POST to _changes is a string, not a []string

Also added better handling of JSON parse errors -- they're now returned
from REST API as 400 status, not 500.

Fixes #293
Bumped version number to 0.94
Merge branch 'master' into stable