Add GET endpoint for arbritrary one-shot value #8

@shroffk I'm just creating this now after our conversation.

As a user of PVWS I would like to have a REST endpoint where I can provide the name of a PV and get its value as a single transaction, rather than subscribing to the web socket and waiting for an update.

At ISIS we have some PVs which rarely ever change, and it would be much easier to just get their value on a page load for example rather than subscribing as it means we can set the websocket up later.

I'll have a go at this as a bit of a personal development exercise!