Added basic_auth() method

[aisrael]

This is patterned after Faraday's own basic_auth() method.

Usage

    client = Cossack::Client.new(TEST_SERVER_URL) do |client|
      client.basic_auth("test", "secret")
    end

which is equivalent setting headers["Authorization"] to "Basic #{Base64.strict_encode(username, password)}"

Test

    client.headers["Authorization"].should_not be_nil
    client.headers["Authorization"].should eq("Basic dGVzdDpzZWNyZXQ=")

(This branch is built upon and also includes the fix for #16.)