Feat/services controllers

[danifromecuador]

In this PR, I've:

• Created a feat/services-controllers branch
• Installed Faraday gem to perform HTTP requests
• Created CoinDeskService to fetch current BTC/USD exchange rate that fetches that data from CoinDesk API
• Created routes and controllers for User and Transaction entities
• Created the following endpoints for the User entity
  • GET /users shows all the users
  • GET /users/1 shows details of the given user
  • POST /users creates a new user
  • PATCH/PUT /users/1 edit or update the balance of the given user when a transaction is done
• Created the following endpoints for the Transaction entity
  • GET /users/1/transactions shows all the transactions of a given user
  • GET /users/1/transactions/1 shows details of the given transaction for the given user
  • POST /users/1/transactions creates a new transaction for the given user
• Update the User balance when a transaction is done
• Fix Rubocop linter errors