HTTP Client-Server model

Deprecate sia-cli in favor of a real Sia daemon (siad) that processes requests from a Sia client (siac, and eventually a web front-end). Running make will produce binaries for both siad and siac.

Limitations/extensions:

• Some configuration values are hardcoded, such as the port number. This is trivial to fix, I just haven't gotten around to it. siad should probably use cobra to facilitate this.
• siac can start siad, but does so by calling exec.Cmd(). This feels like a hack, and it's not cross platform. Worst case, we can just drop this functionality; it's not a huge deal to call siad separately.
• siad needs to more rigorously check its inputs. Specifically, every instance of fmt.Sscan needs error checking.
• I'd like siad to log requests and such, which implies a --log flag for specifying what file to log to.
• The usage information provided by cobra is lacking. For example, the proper usage of the "host" subcommand is siac host [MB] [price] [freezeCoins] [freezeBlocks], but cobra doesn't know that because they aren't flags. I expect this will be a tedious and verbose task.

I'm going to bed.