Add support for std::set serialization

[LukasJoswiak]

To serialize, iterate over the elements and write each one to the output stream. To deserialize, read the elements in and add each one to the set. std::set should take care of ordering correctly, since each object must have had a comparison function to be inserted into the set.

Add unit tests to ensure serialization and deserialization works correctly.

Update README to include std::set support.

See array.hpp for a template on how to serialize and deserialize.