Add Internet Banking support for OpenCart2

[iwat]

1. Objective

This PR implements Internet Banking support for OpenCart 2.0

2. Description of change

This is parent PR of the following PRs: #39, #40, #41, and #49. Those PRs must be merged into this branch before merging this branch to master branch.

3. Quality assurance

🔧 Environments:

• Platform version: OpenCart 2.0.3.1.
• Omise plugin version: Omise-OpenCart 2.0.0.1
• PHP versions: 7.0.17

✏️ Details:

See children PRs.

4. Impact of the change

New payment extension will be installed.

5. Priority of change

Normal.

6. Additional notes

None.