Add qid like ranklib format (#2749)

* add qid for https://github.com/dmlc/xgboost/issues/2748

* change names

* change spaces

* change qid to bst_uint type

* change qid type to size_t

* change qid first to SIZE_MAX

* change qid type from size_t to uint64_t

* update dmlc-core

* fix qids name error

* fix group_ptr_ error

* Style fix

* Add qid handling logic to SparsePage

* New MetaInfo format + backward compatibility fix

Old MetaInfo format (1.0) doesn't contain qid field. We still want to be able
to read from MetaInfo files saved in old format. Also, define a new format
(2.0) that contains the qid field. This way, we can distinguish files that
contain qid and those that do not.

* Update MetaInfo test

* Simply group assignment logic

* Explicitly set qid=nullptr in NativeDataIter

NativeDataIter's callback does not support qid field. Users of NativeDataIter
will need to call setGroup() function separately to set group information.

* Save qids_ in SaveBinary()

* Upgrade dmlc-core submodule

* Add a test for reading qid

* Add contributor

* Check the size of qids_

* Document qid format

doc/get_started/index.md

@@ -5,6 +5,7 @@ on the demo dataset on a binary classification task.
 
 ## Links to Helpful Other Resources
 - See [Installation Guide](../build.md) on how to install xgboost.
+- See [Text Input Format](../input_format.md) on using text format for specifying training/testing data.
 - See [How to pages](../how_to/index.md) on various tips on using xgboost.
 - See [Tutorials](../tutorials/index.md) on tutorials on specific tasks.
 - See [Learning to use XGBoost by Examples](../../demo) for more code examples.