Clean up Cursor and EagerCursor docs and tests

Additionally, there were two small changes to EagerCursor::valid() (only checking key() for null, in accordance with PHP documentation) and next() (remove return statement).
Merge pull request #114 from doctrine/1.0-eager-cursor-cleanup

Clean up Cursor and EagerCursor docs and tests
Make Cursor return values consistent with MongoCursor
Remove code duplication in LoggableCursor and make docs consistent
Merge branch '1.0'