Improve makefile; add conditional SDL include.

Add:
	makefile
		-add .PHONY target, SRC macro that recursively matches all c files in
		src/, and OBJ macro that contains all corresponding bin/*.o files.
		-add SDL.h location detection, which passes an appropriate preprocessor
		macro if it's in subdirectory SDL/ of /usr/include/.

	src/screen.c
		-add conditional SDL.h include.
Add doc/, help/exit commands to shell.

Add:
	doc/
		-folder to contain all documentation files.

	doc/shell_help.txt
		-help file containing a brief summary of the engine's scripting
		language's commands, to be printed in the shell.

	makefile
		-suppress failed killall error.

	src/interpreter/interpreter.h
		-add commands to 'h' and 'e' to print "help" information in the shell,
		and to exit it cleanly (ie, without a simple exit(0)).

	src/interpreter/shell.c
		-add renderHelp().
Update install.sh, add line-types/color to shell.

Add:
	install.sh
		-add documentation header comment, and command to install ncurses.

	makefile
		-set default profile for the gnome-terminal containing the engine's
		shell.

	src/interpreter/shell_graphics.c
		-add color initialization to configureShell().
		-add VisualLine_t typdef'd struct, and line type value macros to allow
		for lines with different colors (eg, an arguments line printed in cyan
		and an error message printed in red).
		-change prompt to empty string for non-command lines.
Generalize printLine(), simplify color selection.

Add:
	src/interpreter/shell_graphics.c
		-create generalized printLine() function, which performs all of the
		conditional checks necessary to print a given line of a given type with
		an appropriate prompt and, if possible, color.

	src/interpreter/shell.c
		-enable use of printLine() in src/interpreter/shell_graphics.c to print
		current line by replacing g_enteringCommand with g_currentLineType,
		which represents the type of the current line based on the object-macros
		in src/interpreter/shell.h.
Create separate utils/graphics modules.

Add:
	src/utils.(c, h)
		-add generalized readFile() function, which reads a file and returns a
		ScannedFile_t struct with all of its content.
		-add ScannedFile_t definition, freeFile().

	src/interpreter/(file_parser, shell).c
		-replace blocks of code that handled file reading with a call to
		utils's readFile().
Mod:
	src/(utils -> graphics).(c, h)
		-rename current utils module to graphics, as it will contain functions
		used exclusively for graphics processing (rasterizing lines, rendering
		shapes, etc.), while utils.c will contain general utility functions.