Fix inconsistent path slashes breaking seen logic

Fixes #8
On windows machines, globs return paths with backslashes, but filepaths
are represented with forward slashes.  This breaks the "seen" tracking,
as `d:\some\path` obviously won't match `d:/some/path`.  Use `path.resolve()`
in the File's `seen` method to normalize slashes in paths before
checking against the "seen" hash, and do the same in the GlobNode's
`findFiles` method.