Allows name changes for existing monitors

[Dunedan]

Currently DogPush doesn't support changing the names of existing monitors. This is apparently, because the ids of remote monitors aren't stored locally, so when changing a monitors name DogPush loses the relation to the remote monitor.
Please store the id of remote monitors and use that as criteria for identifying monitors to allow name changes for existing monitors.

[AMeng]

Renaming is kind of supported after #9, in that you can now delete untracked monitors. So renaming will create a new monitor and delete the old one.