v2.3.0 RC 1

Release Candidate 1 for v2.3.0

• Adds data units (bits/bytes) and data transfer units as first-class citizens
• Adds std::cout support for units with no defined abbreviation (they show up as a combination of SI base units)
• Adds compound assignment operators: +=, -=, /=, *=.
• Adds value() member for accessing underlying type.
• Adds value_type trait, as a synonym for underlying_type.
• Adds definitions for Julian and Gregorian years.

Notes:

• Provisionally removes compiler support for MSVC2013.
• No documentation updates are included in this candidate.