Made more portable, with better behaviour in go1.1+.

By using Go 1.1's path/filepath.SplitList, the code is simpler, has better support for Windows, and should be portable to all platforms supported by go.

In Go 1, the old behaviour of using strings.Split is retained. However, the split is made on os.PathListSeparator, leaving portability up to the go stdlib.

Also in neither case is the code arbitrarily limited to only work on Linux or Windows. This fixes issue #2.