Add "node_modules" to .gitignore. Clarify how head() works. Add pop() method.

I wanted to use this library, but was pretty confused by how `head()` works. The tests did not really clarify anything.

So, I added tests to document how `head()` works and simplified the implementation. Turns out it doesn't affect the list, so I added `pop()` (which does).

Thanks for a really helpful library!