Make state-switching extensible in light

[alexdarling]

Problems:

• Current state-checking format spreads decision-making out across if-else statements. Not feasible to add growing numbers of states.
• Current state switching behavior can only change based on last light state, so complex behaviors are impossible

Proposed Solution:

• Change the state-switching algorithm so it has a planned sequence of states (and timers) that it will fill out.
• Create a history to track which states were used least recently
• Give all decision-making to a central function that uses the plan & history to make decisions each progress() loop.

[alexdarling]

Consolidated the way to change states in 6037b28 and b8ae84b.

[alexdarling]

From 59c8872:

• Instead of just using an if-then statement, updates a queue to track what lights will show up in order.
• switchLight() removed in favor of updateQueue(), and light-switching functionality moved to progress().
• The object now tracks the last meaningful state (any state other than all red lights) and fills its queue based on what that last light was.

It still needs some hardcoding -- the state & lastState attribute determines the next light behavior and the state attribute determines the behavior of 1 light from now. We should consider basing this on user input in the future.

[alexdarling]

This is now good enough to get us to the North-South / East-West traffic setup. Closing this issue as we move on to work on pressure plate detection.