Added the ability to scanning directories singularly and recursively.

As a build step it's often useful to run a linter over your whole
project and see what needs linting. I added the command line option
-recursive to set recursive scanning and also added the requisite
stuff for dealing with directories.

Removed sinful superfluous if block.

Added function name to docstring.

Return nil instead of a slice value.

Fix import style.

Removed recursive flag.

Unexported the IsDir method.

Use filepath.Walk instead of the dilly dallying with building paths beforehand.

Added notes about new directory handling behaviour of golint.