Add automated tests

[jamesrwilliams]

I should write some automated tests to ensure plugin is operating properly with every update. I currently know nothing about automated PHP testing but a good reason to start. Following conditions should be tested as a start:

• On install - Check new DB tables are initialised correctly.
• On uninstall - Check DB tables are removed correctly.
• Test previewing flags
• Test publishing flags
• Test flag groups (full CRUD)
• Test checking group query strings
• Test previewing groups