Ensure that validatePaths can process arrays of globs

I discovered that, whilst you can specify a globbed path containing ** as a String in yuidoc.json (e.g. "path": "**/yui/src/*"), it's not possible to use an array of globbed paths (e.g. "path": [ "**/yui/src/*", "**/example/src/*" ]), or to use the ** pattern in the CLI.

Looking into it, the String works because minimatch is only used to expand the ** pattern if the value is not an array.
Ensuring that paths is always an array and then performing the minimatch on all members of the array has the desired effect.