Add getindex method for Bidiagonal matrix type.

A Bidiagonal matrix is now printed correctly in an interactive session.

Before:
julia> B = Bidiagonal([1,2,3,4],[1,2,3],true)
4x4 Bidiagonal{Int64}:
#undef #undef #undef #undef
#undef #undef #undef #undef
#undef #undef #undef #undef
#undef #undef #undef #undef

After:
julia> B = Bidiagonal([1,2,3,4],[1,2,3],true)
4x4 Bidiagonal{Int64}:
1  1  0  0
0  2  2  0
0  0  3  3
0  0  0  4

julia> B = Bidiagonal([1,2,3,4],[1,2,3],false)
4x4 Bidiagonal{Int64}:
1  0  0  0
1  2  0  0
0  2  3  0
0  0  3  4