New features and modifications

Here are a couple of new features and modifications to meitovexflow. The major ones are:

- moved system and measure logic to separate files. This is related to the second point:
- added automatic width calculation for measures, see @zolaemil 's comment on #21. It works the following way: Each measure with a width attribute in the MEI code will have that measure width; the width of all other measures is calculated on basis of the system width, the voices, start modifiers etc of that measure. This could be something to eventually add to VexFlow, but that might be a little bit more complicated so I added it to the mei2vf code in order to have the feature available. Do you agree with that?
- added support for fermatas
- removed the pgHead and anchoredText code from mei2vf. In order to nonetheless be able to support these elements, I kept the calls of processing functions in the mei2vf code, but emptied these functions. This way, users would be able to provide their own text processing functions (by overwriting the empty functions) and can access the elements in concern as well as data of the mei2vf / VexFlow objects (this is necessary, for example, when an anchored staff text has a horizontal offset (ho) attribute relative to a staff.) It should be possible to remove this way of handling VexFlow-foreign code from mei2vf, but it might need a little bit of extra work. Do you think that should be the way to go?