* Started adding xls format support.
* Add `openpyxl` requirement.
* Add missing serializer class declaration.
* Register serializers with their own format extension(s).
* Sort serializers alphabetically.
* Optional serializer extensions argument.
* Add xls test input files.
* Refactor `io_dict` and `io_utils` for handling correctly local and remote binary files.
* Add `xlrd` requirement for reading old format excel files.
* Remove untestes extensions support.
* Add xls serializer with `.xls`, `.xlsx`, `.xlsm` files support.
* Add support for reading remote binary files.
* Update `README.md`.
Fabio Caccamo