Replace xmp tag with pre tag

Since xmp is obsoleted on HTML5, let's use pre instead of xmp.
Also, this fixes the problem that '&lt;' and '&gt;' didn't work inside xmp tag.
MIDIOutputMap.get should return MIDIOutput

It looks just a typo, but it should be MIDIOutput.
MIDIAccess.inputs/outputs should be readonly

Otherwise users can overwrite it like MIDIACcess.inputs = new myMap().
Merge pull request #86 from toyoshim/pre

Replace xmp tag with pre tag
Merge pull request #88 from toyoshim/readonly

MIDIAccess.inputs/outputs should be readonly
Merge pull request #87 from toyoshim/output

MIDIOutputMap.get should return MIDIOutput
Fix issue #89 (validator errors)
Fixing draft link (was pointing to Audio spec) - fixes issue #92.