Add rows/cols helper functions to Matrix type

These two helper functions (inspired in GNU Octave) can make code more readable sometimes:

    for i=1:rows(A)
        for j=1:cols(A)
            # do something
        end
    end

Could you please consider adding them to the Julia Matrix type?

I had to edit both `doc/stdlib/base.rst` and `doc/helpdb.jl` for the `help()` function to work properly in the REPL. This is by intent?