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
fish calls `$NVM_DIR/nvm`, which in turn is interpreted by bash. Fish is the parent of bash and cannot see changes to environment variables done in bash. So `echo $PATH` will return the unchanged PATH of fish, not bash.

That has been my experience at least, hence the output of the paths and reapplication [nvm.fish#L7](https://github.com/creationix/nvm/pull/579/files?diff=unified#diff-5aa7c1aafb1d525d781f1c83ca10e0c2R7)