add flag to recurse into subdirectories

Was wondering why this did not work. Fixes #58
lintPackage uses build.Import an the second parameter for the source directory was not set to the current directory. With that in place importPaths would work on ../... but not on relative and absolute paths (e.g. "golint/" -> `cannot find package "golint" in any of...`). The current source works on everything besides "..." wildcards in the middle or in the beginning of paths.