Add README-SAMPLES.md and remove exiv2samples.1 (#921)

* Add README-SAMPLES.md and remove exiv2samples.1
* Remove samples man page from CMake
* Add all sample programs to Doxygen.
* Adding links to source code on exiv2.org

Authored-By: Robin Mills <robin@clanmills.com>
Authored-By: Luis Díaz Más <piponazo@gmail.com>

Author: clanmills