Allow to pass certificates as a string parameter

We deploy to heroku which runs ephemeral filesystem that do not allow particular files uploaded, also committing certificates into repository seems like not quite good idea. Everyones recommends [Config Vars](https://devcenter.heroku.com/articles/config-vars) aka environment variables.

I also found that you can easily replace `private_key_file` with string by passing `consumer_secret` thru the options hash:

    Xeroizer::PartnerApplication.new(api_key, secret, nil, ENV['XERO_SSL_CERT'], ENV['XERO_SSL_KEY'], {
      consumer_secret: ENV['XERO_PRIVATE_KEY']
    })