Improve unit tests for contract#validate

- Rename tests with the same name
- Separate test with many assertions on different tests
- Refactor test that is doing integration tests and add mock/stubs to
  make it fit into a unit test
Remove contract unit test that is outdated
Improve unit tests for contract#instantiate

- Separate test with many assertions on different tests
Style improvement of the tests for contract
Upadte README to include the new way to validate responses
Configure rubocop and create a first version of the config file (#27)
Fix problem with "space after method name" (#27)
Fix problem with "Surrounding space missing in default value (#27)
assignment"
Fix problem with "(...) interpreted as grouped expression" (#27)
Fix problem with "Surrounding space missing for '{'" (#27)
Fix problem with "Tab" (#27)
Fix problem with "Space inside square brackets detected" (#27)
Fix problem with "Space missing to the left of '}'" (#27)
Fix problem with "Extra blank line detected" (#27)
Fix problem with "Source files should end with a newline (\n)" (#27)
Activating IndentationWidth check (#27)
Fix problem with "Trailing whitespace detected" (#27)
Fix problem with "Do not use parentheses for method calls with no arguments" (#27)
Fix problem with "has_key? is deprecated in favor of key?" (#27)
Fix problem with: "Keep a blank line before and after private" (#27)