Release 1.3.0

• Add Qty#format and accept custom formatters
• Allow to call Qty() without new to create Qty instances (Qty could be used
  both as a constructor or as a factory)
• Qty#toString only supports to be passed output units as single parameter.
  Former parameters are now deprecated but still supported to not introduce
  a breaking change
• Add mc as alternate definition for prefix "micro"
• Throw error with mmm as unit
• Add rounding optimization