Patches and new features

Here are some more patches and and additions to MEI2VF:

- removed `anchoredText` and `pgHead` functions, see #30. Now, these elements have to be processed separately from MEI2VF. When they occur, MEI2VF currently silently skips them instead of throwing an error. Do you think it would be OK to leave it that way (maybe log a element-not-processed-here message) or should MEI2VF only accept elements it processes? With the first option, it would be possible to use only one document as a basis for text and music; it might facilitate the interaction of text and music processing modules when they are based on the same data.
- added basic support for `dynam` elements, rendered as simple VexFlow annotations; the rendering could be replace by something more sophisticated later (there is a new class in VexFlow which allows to set dynamics/hairpins at timestamps not shared with other tickables, but there are still some discussions about it)
- added tstamp support for `dir` and `dynam` elements and moved the processing logic to a dedicated file
- moved event link calculation from `Converter.js` to a separate file, together with the hairpin, tie and slur classes
- fixed the rendering errors with tie and slur elements across systems
- modified the staff modifier creation in order to ensure that time signatures in a system are always aligned

I haven't done so yet, but I'd like to add copyright headers to the new files. I'm not too experienced in this field. What would be your preferred form regarding author and copyright information?