Many Changes to Client

1. Rename the client package to 'server'
2. Delete the existing client-cli. I don't think the code is gone yet, but it's all commented out. We're getting rid of the cli altogether and instead using 'termbox'. The new termbox frontend has not been built yet.
3. Added a cli interface to running the server, using the library 'cobra'.
4. Added a bunch of configurable options to running the server, leaving room for a config file reader/manager. That's not in this PR, but it's coming soon. (after wallet facelift)
5. Switched all of the server commands to using RPC, with the exception of the wallet stuff becuase that's getting a facelift anyway. The wallet facelift will be the next PR.

Unfortunately Sia's back to being unusable as of this PR, because we need to get the termbox stuff working (after config file PR). Hopefully all of this will be done by the end of the week.
I don't like this. There's no need to recompute the tempdir on every call. If you really want to protect the variable, make a local `var tempdir` and use `TempDir()` as a getter.