Implemented Walk() functionality to recursively walk a rice box.

Have begun to implement the directory walk functionality.

It's heavily based on the original Walk() function in $GOROOT/src/pkg/path/filepath/path.go but replaces key method calls to things like file.Open() with the rice equivalent.

It's not quite ready for merging as it doesn't currently work with embedded boxes.
This is because virtualDir.readdir() in virtual.go is not yet implemented.
Currently it just throws a ErrNotImplemented error when it hits that method.

I can have a go at implementing the virtualDir.readdir() method however I have run out of time for today.
I thought i'd commit what's  done so far to allow you to sanity check/critique it before I continue.

Ref to issue #12