custom Padding for correct alignment of time signatures in multi-instrument setting

Custom padding can be passed as second parameter to Vex.Flow.TimeSignature 
to allow the correct alignment of time signatures in a multi-instrument setting (particularly when using percussion clefs/staves in conjunction with staves that have different clefs).

- [x] implemented feature
- [x] added documentation
- [x] added unit test

Usage: 
``` javascript
var stave = new Vex.Flow.Stave(15, 0, 300);
stave.addClef("percussion"); 
// passing the custom padding as second parameter (in pixels)
stave.addTimeSignature("4/4", 25); 
```