Add Code framework

Add empty framework for Code class.  The Code class decodes mnemonics
and returns the corresponding bit patterns as strings.

Add: Code.java
Implement Code class
Add CommandType enumeration

Add CommandType enumeration to represent type of commands (a-, c-, or l-
command).

Add: CommandType.java
Add Parser framework

Add empty framework for Parser class.

Add: Parser.java
Implement Parser class
Add method to format number as binary to Code

Add method to Code class to format a number as a 15-bit, 0-padded binary
representation.
Add Assembler class

Add Assembler class to handle translation of assembly files to machine
language.  Current implementation only correctly translates assembly
files that do not contain symbols.

Add: Assembler.java
Add Program class

Add Program class as main entry point.  Program class reads command-line
arguments, prepares files, and creates an Assembler to translate source
file.

Add: Program.java
Add SymbolTable class

Add SymbolTable class for recording symbols used in assembly programs.

Add: SymbolTable.java
Handle symbols in Assembler

Modify assembler to correctly handle symbols as well as numeric
addresses.