303 nvm in fish

Hi,

Related to #303 , I use fish myself and love that I can run `npm install -g <...>` with nvm --> fish nvm was needed.

This is simply a wrapper around nvm for fish. I have commented my commits as much as possible to help understand my thought process. I hope others with fish can test this (tests in vagrant worked for me).  
A wrapper allows nvm to change below without (hopefully) affecting functionality in fish.

I'm open to critical feedback or improvement suggestions.

Cheers