macd: widen to accept multi-column TimeArray

[iblislin]

julia> macd(ohlc["Open", "Close"])
467x6 TimeSeries.TimeArray{Float64,2,Date,Array{Float64,2}} 2000-02-18 to 2001-12-31

             Open_macd  Close_macd  Open_dif  Close_dif  Open_signal  Close_signal  
2000-02-18 | 0.5907     -0.0472     3.3076    3.5339     2.717        3.5811        
2000-02-22 | 0.1396     -0.1375     2.8914    3.4092     2.7518       3.5467        
2000-02-23 | 0.0229     -0.0636     2.7805    3.4672     2.7576       3.5308        
2000-02-24 | 0.1838     -0.1131     2.9873    3.3894     2.8035       3.5026        
⋮
2001-12-26 | -0.1753    -0.1494     0.3343    0.3244     0.5096       0.4738        
2001-12-27 | -0.1333    -0.0861     0.3431    0.3661     0.4763       0.4523        
2001-12-28 | -0.0794    -0.0231     0.3771    0.4234     0.4565       0.4465        
2001-12-31 | -0.0111    -0.0203     0.4425    0.4212     0.4537       0.4414