Make it safe to change host.
Add read lock around using host.
Remove the lock.

I kindly ask that you set your host and then never change it.
Remove public set method as Host is public now.
Change secure to scheme.

* default scheme to http
* make it public so it can be changed
gofmt.
Merge pull request #6 from jnunemaker/safe-host

Move host (formerly Endpoint) to client