Extracts resquests into their own class

There is no need for the paymill module to know about the internals of a
request, that can be hidden away in its own class
Moves https setup into a private method
Request related classes now have a single responsibility
Applying Sandi Metz rules
Payments in sync with API
Adds Payments to the README
Removes some data dependencies
Removes the data dependency on response
Removes more data dependencies
Refactors tests
Removes test file
Merge pull request #30 from zamith/master

Requests refactor and debit card payments