Add samples to CMake build

[rotolof]

This will add the examples to the CMake build.

mkdir build && cd build
cmake ..
# This will build everything
make
# This will build only the library
make JetsonGPIO
# This will build the examples
make examples

• Examples will be configured/built unless -DBUILD_EXAMPLES=OFF is passed to cmake.
  We can choose to enforce the opposite behavior by putting this instead:

  option(BUILD_EXAMPLES "Build examples" OFF)

  So examples will be configured /built only if we pass -DBUILD_EXAMPLES=ON to cmake.

• I didn't add samples/test_all_apis.cpp to the examples since it's already there as a test.

• At the moment, the generated binaries will not be installed.

[pjueon]

Thank you very much.
For the default value of BUILD_EXAMPLES, I prefer OFF.
I set it to OFF and updated the README file.