Add sample billing config

Dependencies

• [2] MJ6R42RC Utility methods for reading key & cert data.
• [3] 6L5BK5EH Use generic SMTP rather than Sendmail-specific mail client.
• [4] V2VDN77H Enable postgres configuration via environment variable for Heroku.
• [5] LCBJULKE Fix swapped default and key in QConfig.
• [6] HMDM3B55 Implement core of payments/billing infrastructure.
• [*] QO4NFWIY Added sample config file.
• [*] M3KUPGZK Add invitation email template.

Change contents

• edit in conf/aftok.cfg.example at line 30
  [9.308]

  [9.308]
  + billing {

  + network = "test"

  + signingKeyFile = "/etc/aftok/aftok.bip70.key.pem"

  + certsFile = "/etc/aftok/aftok.bip70-chain.cert.pem"

  + exchangeRateServiceURI = "https://blockchain.info/ticker"

  + }

  +
• replacement in server/Aftok/QConfig.hs at line 63
  [3.572]→[2.397:433](∅→∅)
  − <*> readBillingConfig cfg
  [3.572]

  [3.3]
  + <*> (readBillingConfig $ C.subconfig "billing" cfg)