Refactor DocReader to accept diff doc types.
Refactor DocReader to accept diff doc types.

Prepare to read into something other than a Doc object from
DefaultDocReader.

rectang/lucy,apache/lucy,nwellnhof/lucy,nwellnhof/lucy,rectang/lucy,rbevers/lucy,apache/lucy,apache/lucy,nwellnhof/lucy,apache/lucy,rectang/lucy,nwellnhof/lucy,rectang/lucy,apache/lucy,rbevers/lucy,rbevers/lucy,nwellnhof/lucy,rectang/lucy,rbevers/lucy,rbevers/lucy,apache/lucy,rectang/lucy,rectang/lucy,nwellnhof/lucy,rbevers/lucy,rbevers/lucy,nwellnhof/lucy,apache/lucy