`Store.MaxChannels()` => `CanStore()`.

Simplify `NoStore`.
Add failing handshake test with channel IDs.

Remove redundant `TestHeloReset` test.
Extract handshake logic into `Worker.handshake()`.

Ensure `handshake()` resets unknown device IDs if channel IDs are
specified.
Merge pull request #119 from mozilla-services/bug/existing-handshake

Ensure `Store.Exists()` looks up the correct device ID
Merge branch 'dev' of github.com:mozilla-services/pushgo into bug/opaqueCHIDs

Conflicts:
	src/github.com/mozilla-services/pushgo/simplepush/api_test.go
	src/github.com/mozilla-services/pushgo/simplepush/no_store.go
	src/github.com/mozilla-services/pushgo/simplepush/server_test.go
	src/github.com/mozilla-services/pushgo/simplepush/worker.go