Add custom errors and more tests

* Added new `Activity` assert.
* Added custom `AuthySmsNotSentError`.
* Added custom `AuthyUserNotFoundError`.
* Added tests for all asserts.
* Added tests for all errors.
* Moved errors to `lib/`.
* Removed listing `countries`.
* Updated asserts names.
* Updated mocks based on actual responses (not sandbox).
* Updated `parseRawInput` to return `countryCallingCode` as a number if valid.
* Updated debug@2.1.0.
* Updated google-libphonenumber@0.1.1.
* Updated nock@0.52.2.
* Updated validator.js-asserts@0.0.3.