Added standalone build instructions for browser use

I added standalone build instructions for browser use. There is readme documentation for using the npm run script added(`npm run build_standalone`) or completely manual.

I also added some classes to easily style and differentiate between the X and Y axis. Moved the labels out a bit further so they don't overlap the ticks.
Installing browserify won't be needed, as `npm install` takes care of this. I think we can take this "or" alternative out, and just mention the extra `run` script you added. Perhaps we can also shorten the name, `build_standalone` is a bit awkward to type.. just `standalone`?