Remove curb-fu once and for all

Related: https://github.com/fastly/fastly-ruby/pull/41

Curb does indeed suck. Much of the complexity around the client/fetcher stuff is due to it. 

This PR removes curb-fu entirely from fastly-ruby and adds unit tests around fastly/client.rb

Changes I saw fit to make in the process:
- Add KeyAuthRequired over just using AuthRequired
- GET 404s no longer return nil and instead raise a `Fastly::Error`
- Fixes stats_test.rb. I don't even know what was going on there.

This also makes #36 irrelevant
For Ruby 2.0 and newer we should `@http.start` to take advantage of persistent connections.

For Ruby 1.9 and older we can use my library net-http-persistent which will manage persistent connections for both new and old ruby.

I think Ruby 2.2 will very SSL connections here, but we don't do any setup for that for existing versions.

We can use my library for that, or just take some code from it, especially [setting up the certificate store](https://github.com/drbrain/net-http-persistent/blob/master/lib/net/http/persistent.rb#L1232-L1238) and setting the verify mode to VERIFY_PEER.