Added support for signing requests with Aws::CloudSearcDomain::Client.

You can construct a client with or without credentials. When credentials
are present, then the request is signed with a version 4 signature.
The region name is extracted from the endoint.
Added unit tests and changelog entry for signed Aws::CloudSearchDomain::Client requests.

Also, DRY'd up unit tests by disabling external credential loading
in one place, the spec_helper.rb.
Disabled default proxies when requesting credentials from EC2.

Net::HTTP automatically creates a proxy if the HTTP proxy
environment variable is set. By passing a `nil` value to
`Net::HTTP.new` for the `proxy_address`, Net::HTTP skips this.
Whitespace fix.
Corrected format of deprecation warning.
Enabled Travis deploys to GitHub of the API docs as a release artifact.
Doc updates for Aws::CloudSearch.