Add a parse_file method

- Should return an ast corresponding to the assembly file
- Also refactored main slightly
Add jump table; for now, just print the file
Add tests target to scons; use lit.py

- Import lit from llvm, and add LLVM license to readme
- 'scons' will build, but not test
  'scons test' will test
  'scosn test VERBOSE' will print the errors (if any)

- Add two tests:
  test-empty for empty files; currently fails
  usage, to make sure the usage info is printed