Oops, fix crash in the new retry code

It shouldn't be clearing _request, just _connection.
Bump version to 0.61 for alpha2 candidate.
Fix a crash with replication after closing a database

TDDatabase wasn't unregistering itself from replication notifications in its -dealloc method, so after a database has been dealloced any notification from any replication would likely cause an exception or crash.
Fixed missing response for some failing REST requests

Nasty bug fixed: TDRouter would react to some errors (like a request to a nonexistent database, or an invalid document ID) without returning an HTTP response, so the caller's request would fall on the floor and eventually time out.

Also improved the TDRouter unit tests to catch this type of problem.
Replicator now properly escapes slashes in doc IDs
Offline support

TDReplicator detects when the remote host becomes reachable/unreachable, usually as a result of the device going on/offline. Replication is stopped when offline and restarted when back online.
Make replicator active/idle status accurate

The 'active' property of TDReplicator wasn't being updated correctly. This in turn broke the status being reported in the _active_tasks REST API.
Persistent replications ('_replicator' database)

Not really tested yet.
Get persistent replications working with REST API

And updated Mac demo to use them.
Change iOS demo app to use persistent replications
Replication bug fixes

* Persistent replications weren't restarting on app launch (oops).
* Fix exception when persistent replication is deleted.
* Wait longer (5sec) before saving checkpoint to remote server.
Allow restart of a persistent replication

Deleting the _replicator_state property of a replicator document starts a new replication.
(This is also supported by CouchDB, but undocumented.)
This makes persistent non-continuous replications more useful.
Overhauled checkpointing for pull replications

Added TDSequenceMap to keep track of the latest sequence ID we can checkpoint.
(Should be used for push replications too, but I haven't done that work yet.)
Pull deleted revisions after live ones

Optimization for pull replication: prioritize pulling non-deleted revisions. This doesn't make the total time shorter, but it makes visible stuff show up sooner, especially in a case like the first launch of GrocerySync, where there are a lot of older documents that have been deleted already.
Fix a leak and appease the static analyzer
Removed a bogus build setting in the Mac demo target

It was just causing a harmless link warning.
Speculative fix for replication issue with dup attachments

If a doc has two attachments with identical contents, TDMultipartDownloader will think there are fewer attachments than there really are (because it keys them by SHA-1 digest). This isn't a problem except for a sanity-check that was too strict. Loosened it up.
Fixed some issues with filtered push

* When a revision is added while a continuous push is active, the filter block was called with nil properties (fixes issue #47).
* In a filtered push the previous revision IDs of a document wouldn't be sent over, so the history got lost.
Avoid conflicts saving checkpoints

Replicators now avoid saving a checkpoint while a previous save is still in progress.
Instead they wait till the save finishes, then start the new one.
That way the second save will have the correct revision ID to replace.
Fixes #48.
Fixed 403 error POSTing to _replicator db