Add unit testing for every function and integrate with Travis CI

[lewisgoddard]

Primary

• New unit tests are added automatically.
• Test against MySQL and MariaDB

Functions

• function mysqli_array_build($connection, $sql)
• function drop_table($connection, $settings, $table)
• function mysqli_fetch_once($connection, $sql)
• function peer_access($connection, $settings, $time, $peer)
• function peer_completed($connection, $settings, $peer)
• function peer_delete($connection, $settings, $peer)
• function peer_new($connection, $settings, $time, $peer)
• function maybe_binary_to_hex($binary)
• function task($connection, $settings, $task, $value)
• function tracker_allowed($connection, $settings)
• function tracker_clean($connection, $settings, $time)

Impossible

As it stands, there are some functions whose composition is not lended to unit testing. These should probably be fixed later, and we can work around the super-globals for now.

• optimize_table has no current failure states.
• tracker_error contains an exit expression.

[lewisgoddard]

Apparently we can't test against MariaDB and MySQL right now, so we'll stick the the older one and work from there.

[lewisgoddard]

Optimize table has now become a task instead.