1. fixed a couple bugs with config file reading.
2. made cmd.Serve a global variable so tests could inspect it.
3. added table style testing for pilosa server
4. added the ability to stop server programmatically which also causes the cobra
command running it to return.
Now that the default data dir is defined by the flag, we don't need to set it
explicitly if it isn't set. We also stop reading the config file explicitly in
server.go since viper will read it - we do need to define all the config options
though before the config file will work properly.
