Exclude tombstone resources when getting a resource's children
Ensure generated link headers include an anchor parameter as needed

Fixes https://www.pivotaltracker.com/story/show/81018546
Merge pull request #561 from fcrepo4/exclude-tombstones

Exclude tombstone resources when getting a resource's children
Merge pull request #563 from fcrepo4/link-anchor

Ensure generated link headers include an anchor parameter as needed
Rename GraphSubjects to idConverter