List payment channel services

[rllola]

How should we load payment channel services ?

• Hardcoded services available (so we verify them too)
• List them in a json file (can easily add service)
• Let user add services with a form (maybe unsafe)

[rllola]

This PR add a temporary list in constants.js (https://github.com/BitcoinAmiens/dogecoin-spv-node/pull/63)

In the future it could be better to have them in a .json file and just pass it the name of the file in the command line.