js: Add callingConvention setting

For now it defaults to the existing behavior "by-position-only" and changing it
has no effect.
js: Don't include params if method is called without arguments
js: Add by-position-or-by-name calling convention

Fixes #699
Merge pull request #746 from jodal/feature/js-calling-convention

Add by-name calling convention to Mopidy.js