Patch for beefsack/go-astar Issue 1, moving goal condition check in asta...

... astar.go to preserve optimality

Moved the check for goal state out of the neighbour expansion loop.  

This patch was designed to fix https://github.com/beefsack/go-astar/issues/1

Can we change these to `//` style comments?  They're practically all you find in the official Go source (eg. http://golang.org/src/pkg/encoding/xml/xml.go)