Mixed value types

There are no checks that the value element of a Timestamp share the same type when they are aggregated inside a normal array. This allows some liberties in constructing these arrays:

julia> integer = Timestamp(Date(2002,1,1), 12)
2002-01-01 | 12

julia> string = Timestamp(Date(2002,1,2), "twelve")
2002-01-02 | twelve

julia> vcat(clstamp, integer, string)
502-element Array{Timestamps.Timestamp{T},1}:
2000-01-03 | 111.94
2000-01-04 | 102.5
2000-01-05 | 104.0
2000-01-06 | 95.0
2000-01-07 | 99.5
2000-01-10 | 97.75
2000-01-11 | 92.75
⋮
2001-12-24 | 21.36
2001-12-26 | 21.49
2001-12-27 | 22.07
2001-12-28 | 22.43
2001-12-31 | 21.9
2002-01-01 | 12
2002-01-02 | twelve