* support dygraph to static graph for simple case.

* add test for dygraph API recognition. test=develop

* support basic to_variable api. test=develop

* update dict: dygraph_class_to_static_api

* add all tests of dygraph api. test=develop

* use gast/astor instead of ast/codegen for the compatibility of PY2 and PY3. test=develop

* add arg 'num_flatten_dims' for fc ast node. test=develop

* Modify names of class by Camel-Case.