* add more unitest for ABI compatibility

* add more unittest

* refine warning style

* support compile multi custom ops in same time

* fix not import paddle in unittest

* fix typo

* add more unittest

* add comment for details

* How to write custom op needs to follow framework OP spec.
* Package fluid_framework.so and headers into whl.
* Add paddle.sysconfig.get_include() and paddle.sysconfig.get_lib() to get include dir and lib dir.
* Export some C-APIs to merge OpInfo between core.so and custom_op.so.
* Add unit testing.
* Update API.spec.