Path Planning Starter

Why

1. Got a bit tired of Udacity projects starter code mess (usually, its a monster function with lots of variables, lots of duplication and lots of confusion). Decided that sharing a cleaner starter code will benefit all the SDC students.
2. I didn't find the code, which was shown in Walkthrough

What

Implemented 2 basic path planners:

1. KeepLanePathPlanner, that just goes straight with fixed speed, keeping lane
2. SimpleSplineBasedPlanner, this is a planner shown in Walkthrough video. It can accelerate/deaccelerate and switch lane to the left.

Later I will put inside some skeleton implementations of PathPlanner interface, which will use state machines/cost functions/etc.

Principles, I tried to stick to

1. Clean code, SOLID
2. Reasonable usage of modern C++

Prerequisites

Use those from the original starter project.

TODO

1. Clean up HighwayMap class
2. Enforce consistent code-style