Relative doc links

I wanted to be able to generate the docs and view them locally using a tool like [`node-static`](https://github.com/cloudhead/node-static) and all the links were relative to `/gcloud-node/` because we assume this is being served from github pages at that relative url. This change makes them relative to whatever folder they're already in, so you can do something like:

``` sh
npm run docs
cd docs
static .
# serving "." at http://127.0.0.1:8080
```
Open browser to http://127.0.0.1:8080 and view them there. 

This also means I can just git push the **whole repo** to my `gh-pages` branch and my docs are served up at:

http://ryanseys.com/gcloud-node/docs/#/

Note: the `/docs/` in the url now because the whole repo is served so we have to navigate to that sub folder for the docs but this is not neccesary, we can still do the sub-module magic to serve at the plain `/gcloud-node` for our repo if we want and with this PR we should be able to switch between them effortlessly.