Stubbing out the basic structure for the first pass of the assembler.
Punched is_numeric? into the Ruby String class.
Assembler:
  Added call to addressify the labels (the first pass)
  Enhanced assemble_a_command to be sensitive to numeric constants AND symbols, eg @foo.
Parser:
  Added rewind method to reset the cursor to the start.
SymbolTable:
  Added internal hashtable of predefined symbols, per spec.
  Stubbed out address_of to return 0, for now.