Add multiplex operator

implement framework of seq_pool_op and its unitest

adding doc for comparing cluster bootstrapping tools

Change LoDTensorDesc::dims from int32 to int64

make RecurrentOp's backward compatible with framework

Fix progma_once error

Change namespace of pybind.cc to pybind

Remove `numel` field in tensor

* It is duplicated with `dim_`. We can use `dim_` to calculate `numel`
  everytime. It does not cost too much.
* `numel` is not initialized by constructor. Also, `numel` is hard to
  synchronize with `dim_`.

So just remove it.

Elementwise operator add/sub/mul/div

Fix bug in cc_library, when merging several libraries into one on Linux.

Add bool type for attribute and use it in dropout_op.

Pass LoD from input to output in the operators.

* update faq

* follow comments

Add `BlockDesc` and `ProgramDesc` to framework.proto

Fix linking error on mac.