Change absctract_buffer behavior

* methods data return nullptr when buffer length == 0,
* mutable_buffer now uses count method from abstract_buffer.
Increases tests coverage

* clean test_buffer.cpp,
* update test_mutable_buffer.cpp to cover count and push_back
  methods.
Add gmock (v1.7.0) to dependencies

* Update Test.mk and build_tests.mk to compile gmock