Add node-ip type definitions

Add node-ip type definitions file with a test script.
Add empty NodeBuffer interface

The empty NodeBuffer interface acts as a placeholder if node's type
definitions file isn't referenced. This is to hopefully stop the Travis
CI build error.
Rename node-ip directory to ip

Rename node-ip to ip so the folder name matches the library name on npm.
Merge pull request #2844 from CodeAnimal/node-ip

Add node-ip type definitions