Add PHP Code Sniffer 3.x to the composer package.

[guzzilar]

⚠️ This pull request requires #87 to be merged first.

1. Objective

Omise-PHP recently has been developed using the PSR Code Standards as a based code-style guide, but still, many parts of the code are still left untouched.

This pull request is introducing an automate tool to scan through the whole project finding those left-over code so the developers can provide a better code maintenance and code consistency.

Note 1: This pull request is not to introduce the change of those code styling but to introduce a tool.

Note 2: As in the ruleset, there are 2 excluded rules. It is because originally the project hadn't been developed based on those 2 rules, and it would require a lot of changes on both core code of the library and the interface (like, namespace).
So I decided to pull it off for now, it will be applied back again at Omise-PHP v3.0.

Note 3: This PHP Code Sniffer can be introduced as a one step for the CircleCI to automate it, but as from the note [1], I think it would be a good idea to provide a code-cleaning first, before add this phpcs into the CircleCI step.

To install and test the script

You can install the package via Composer by executing the following code: composer update.
Then, run ./vendor/bin/phpcs to scan the project.