Refactoring Commands

Now that we have the `commands` package set up, it's time to move everything over to it. This PR is currently not ready for merge, just setting it up for CR.

The basic scope of this PR is as follows:
* Set up transports based on `commands` package (HTTP RPC server/client, CLI command handling)
* Refactor existing commands for the new API (won't be a 1:1 mapping, commands will change)
* Make changes to `commands` package when necessary
@mappum could make it harder to vendor this if we import ipfs-specific packages in the `commands` lib. Perhaps this root command can be passed in?

``` go
func NewHandler(r commands.Command) Handler {
  return Handler{Root: r}
}