Make Factory API the default

[0xfe]

Now that we have a fully functional Factory API in src/factory.js, update all documentation to use it.

[rvilarl]

@0xfe is there anything special that you want to do in the documentation of Factory (ie.: examples?)

[0xfe]

I wanted to change all the documentation to use Factory instead of raw vexflow classes. There's a bunch of places this needs to be done: e.g., https://github.com/0xfe/vexflow/wiki/The-VexFlow-Tutorial