Refactor Node.gitignore with comments

The "build" directory is lost in this refactor, as many JS projects
do wish to include a minified/concatenated version of their source
in the "build" folder in source control.
Clarify node_modules comment

Now linking to the official npm FAQ entry (with tl;dr) rather than straight to mikeal's blog post
Add build/Release to Node gitignore

This re-covers the http://nodejs.org/api/addons.html case for which the "build" directory was added in 00686415c4, but doesn't break cross-environment projects that use the build directory for including concatenated/minified source. Mazel tov!
Add `coverage` directory, rename coverage heading
Add .grunt directory
Node.gitignore: correct term for instrumentation
Merge branch 'master' of https://github.com/github/gitignore into stuartpb-node-patch

Conflicts:
	Node.gitignore
Reduce comments
Merge pull request #913 from stuartpb/node-patch

Refactor Node.gitignore with comments