Change Makefile so it's easier

Now you just type `make book` or `make serve`, without having to add
GITBOOK=`pwd`/node_modules/.bin/gitbook.

Also, now npm installs it automatically on first run.
Why did we cd into stage in the first place

This works perfectly, as far as I can tell.
Merge pull request #328 from GBGamer/makefile

Change Makefile so it's easier