Boundaries update! Finally!
* Added class Boundary to define boundaries (walls or whatnot). Boundaries for each level are defined in the cooresponding map data file. In the future, I'd like to procedurally generate the boundaries with the input of the terrain, but this is more complicated than it seems.
* Added Boundary vector to currentMap (a TerrainMap instance which is the currently loaded map)
* Added physics calculations for collisions with boundaries
* I'm not sure if it was this commit, but I previously set up SDL_Mixer to soon play background music and sound effects. The background music of each level will be stored in the TerrainMap instance of that level and could potentially be specified in the map data file.

Note: Boundary collision calculations aren't bug-free just yet:
* I'm going to incorporate the base_posX, base_posY, and base_radius to calculate collisions with the player's base (their feet).
* Also, currently the collision detection is checking for a badX and badY at the same time. This prevents moving in both directions when maybe there should just be one direction prevented. I'm going to change this in the next commit.