Change filename for more common protocol naming

[utzig]

I would suggest renaming newtmgr.js to mcumgr.js. Newtmgr was the original name used by Mynewt, while MCUmgr is the naming employed in the multi-platform support efforts, which should eventually be more common (if not already).

[boogie]

Next commit will fix it.

[boogie]

Fixed.