Fix show method for the MMAP model

[GiggleLiu]

fix #93

julia> mmap = MMAPModel(model, evidence=Dict(1 => 1, 8 => 1), queryvars=[4, 7])
MMAPModel{Int64, Array{Float64}}
variables: 1 (evidence → 1), 4, 7, 8 (evidence → 1)
query variables: 4, 7
contraction time = 2^29.356, space = 2^20.0, read-write = 2^22.673