add SequenceConcatLayer in trainer_config_helpers

Filter the empty sentences.

fix issue in create custom reader in design doc

Remove sequence information in GradientPrinter.
Fix #1376

create buffered data reader decorator and tests

Unify print layer, ValuePrinter evaluator.
Fix #1378

Unify Evaluator::create method

hide parse_config

Checking whether the input graident ptr is null in seqconcat layer.

* it is not necessary, because user could print sequence info
  by ValuePrinter.

* Simplify Evaluator::create logic. Always use ClassRegister to
  create Evaluator.

add data provider design doc proposal

travis ci: cache brew directory

Fixes #1365

add v2-layer

Draft for new API

Add default mkl dir in cmake