add basic auth

[giwa]

Thanks to #11, we can use basic auth by customizing Faraday.connection. Since BasicAuthentication is provided by Kintone platform, it's worth to provide a method from the library. Otherwise, it should be documented like below at least. I'm the one who could not figure out how to handle basic auth lol.

# Basic authentication
api = Kintone::Api.new("devhopjsm.cybozu.com", "token") do |connection|
     connection.basic_auth("user", "password")
end

[giwa]

@jue58 ping

[sy250]

#11 マージはされましたが、リリースされていないようです。
#11 のパッチを当てることで、#15 のコードでBasic認証に対応できます。