Make MIDIInputMap and MIDIOutputMap more compatible with ES6 Map

For now, forEach() is one available function to enumerate ports.
But, it is designed to be compatible with ES6 Map and iterator.
We can switch to use iterators without any compatibility breakage
once ES6 is ready.
Merge pull request #98 from toyoshim/compat_map

Make MIDIInputMap and MIDIOutputMap more compatible with ES6 Map