When printing matrices, replace zeros by dots

This makes it easier to read many matrices, especially those which
are relatively sparse.

Before:

    julia> matrix(QQ, [42 0 0; 0 42 0; 0 0 42])
    [42//1    0//1    0//1]
    [ 0//1   42//1    0//1]
    [ 0//1    0//1   42//1]

After:

    julia> matrix(QQ, [42 0 0; 0 42 0; 0 0 42])
    [42//1       .       .]
    [    .   42//1       .]
    [    .       .   42//1]