New unit system <boost/units/systems/information.hpp>

Defines base units and system units for: bit, byte, nat, hartley and shannon,
and scale units for binary prefixes kibi, mebi, gibi, tebi, pebi, exbi.
I would add as well zebi and yobi prefixes for completeness.

```cpp
BOOST_UNITS_INFOSYS_PREFIX(70, zebi);
BOOST_UNITS_INFOSYS_PREFIX(80, yobi);
```