Store each watch config in its own directory, saves rewriting the whole json file each time

[dgtlmoon]

todo

• use a checksum or string comparison lookup of the watch dicts to know if they should be updated
• move the file/IO stuff out of 'store.py' into 'storage.py' perhaps, so it could be used with other systems? (other document storage backends)

Advantages

• Lower memory usage when using json.dumps
• MUCH lower disk-io
• In the future a command-line can be run against just a single watch config
• Easily import and/or copy configs from somewhere else, just dump them into the datastore directory