Add changeMarker function to library

A couple of comments:

1. This is the first pull request I've ever made (as well as the first open source contribution I've ever made). Hopefully the code style is acceptable and whatnot. Feel free to hit me up with any advice / constructive criticism.

2. Disregard commit e48b967.

3. Motivation for the fork and pull request: I've been using your truly awesome library for a project for my internship. I've needed to dynamically update markers while maintaining their order, so invoking removeMarker and addMarker just wouldn't do it (it changed the order of markers). To solve that, I wrote up a changeMarker method.

4. There'd be lots of shared code between createMarker and changeMarker, so I refactored it into an initializeMarker helper function.