Remove external dependencies

On my pending [pull-request](https://github.com/Automattic/node-canvas/pull/571) I was able to compile Cairo statically inside node-canvas, so it's not needed to have it installed globally. I can be able to cherry pick the changes and do a new pull-requests with them if there are willingness to merge it, because I think it can help not only a lot of newcomers (the error when Cairo is not found is somewhat cryptic, specially since people would not be aware that node-canvas is a tree dependency of the module they are installing) but also fix some problems regarding to its installation.