add Server.attach method

This method does what engine.attach use to do but is now on the
instantiated server instance. This makes it possible to create engine.io
servers without yet attaching them to any active http server.

When creating a project which will server static files/pages alongside
engine.io on the same domain/process this makes it easier to separate
logic into files without having to pass around the http server.
remove unused require
remove lib/index.js

Needless level of indirection which blocks actually reading the
important parts of the code.
make require('engine.io')() return a new Server instance

fixes #212
lib: remove unused file