Version 2.1

• Set USE_SHELL_API to default to 1. This better supports
  different flavors of linux.
• INSTALL can now be configured on the command line.
• Parsing and formatting fractional seconds can now handle
  super-second durations such as microfortnights.
• Formatting and parsing rewritten, moved into date.h.
• iOS support added.
• Added trivial default constructors for most calendar
  types.
• Add unary operators + and - to year.
• Add to_stream.
• Miscellaneous bug fixes.