Fix graphical glitch around next piece preview.

The preview pane bitmap was not initialized; on some systems this caused
graphical glitches in the next piece preview. It is now cleared before
drawing the piece to a color with no alpha channel.
Write a build system.

The build system is autoconf and make. gitignore has been updated, and a
cleaning script provided because the working directory can get ugly.
Change member list for stage_t and add color.

The features of the member list in stage_t have been changed to ones that
make more sense. The stage_t enum should indicate only stages of the game,
without regard to the status of the screen or program at that point (e.g.
PAUSED).

A color "FADED" is added to the palette. This will be used for game over
screens as a tint on the shot of the just-lost game.
Clean up screen state switching and add score.

A game over score screen has been added, and in the implementation, the
tracking for the different screen states (menu, in game, score screen) has
been improved.

Instead of a boolean for in_menu, the mixer keeps track of a screen_state
variable, holding enums for MENU, GAME, or SCORE.
Add keyboard controls.

Arrow keys are now supported alongside WASD, and P can be used for pause.
Merge branch 'master' of https://github.com/PaytonTurnage/Metris