v1.4.5 - fixes for negative numbers

• forth: / now uses floored signed division
• forth: bugfix for negative number parsing (2- was
  interpreted as -20)
• forth: made 2/ sign-extend negative numbers
• forth: added spaces source >in 2over 2swap m+ m*
sm/rem fm/mod dabs dnegate s>d
• forth: changed word order of double-cell integers
• vi: backspace on start of line now doesn't join lines
• doc: mention in tutorial that default is hexadecimal

Thanks to Christian Johansson and polluks for suggestions!