Add showcompact method for vectors of Number's

Currently there is a peculiarity that `showcompact` applied to a floating point number produces a compact representation, similar to the representation the `print_matrix` method.  The `showcompact` method for a vector of numbers does switch to a horizontal orientation but uses the full precision.

This pull request adds a `showcompact` method for vectors of numbers to use showcompact on each number.  It doesn't use `tty_cols()` and hence could produce very long lines on long vectors.
```jl
julia> r5 = rand(5)
5-element Array{Float64,1}:
 0.381319
 0.337257
 0.409786
 0.9746  
 0.679875

julia> showcompact(r5)
[0.381319,0.337257,0.409786,0.9746,0.679875]
```

I can understand if a more sophisticated version is desired but I do think that the current situation of writing full precision representations in `showcompact` should be changed.