Move toward the actual REST api w/ json

This patch moves us more toward the rest API in the spec. ping and container
creation are now restificated (with the small caveat that create is currently a
sync response, I will implement async ASAP).

I've defined several helpers (/lxd/response.go) for generating the API-style
response json objects. I've also defined (/client.go) a Response that
represents a response from the API.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Should be every time backward compatibility is broken.

The api_compat level should also be stored somewhere in there I guess.