Changes in Version 0.2-0

  o added support for instrumental variables regression
    (two-stage least squares) via formulas like
      dynlm(y ~ x1 + x2 | z1 + z2 + z3, data = mydata)
    where z1, z2, z3 are the instruments.
  
  o enabled specification of multiple lags via formulas like
      dynlm(y ~ L(x, 0:4), data = mydata)
    where y is regressed on x and lags 1 through 4 of x.

  o fixed bug in time properties when there were leading
    NAs in the data