Rename NextResult -> NextPath

See discussion in #92.
Simplify Nexter interface

This change allows a Nexter to be used in the same manner as a scanner
using a for graph.Next(it) {} construction.

It is important that graph.Next(it) and any associated it.Result() calls
operate on the same iterator.
Merge branch 'master' into nexter

Conflicts:
	graph/leveldb/all_iterator.go
	graph/leveldb/iterator.go
	graph/memstore/triplestore.go
	query/gremlin/finals.go
Sync iteration calls to Nexter interface changes
Merge pull request #93 from kortschak/nexter

Simplify the Nexter interface