expose query units on the payload
Data types with units can no longer change units

- set corrct unit type when parsing json, do not infer from
  parent
- simplify ConvertableUnits to prevent unit switching
- extract ConvertableUnit specific tests to a single spec
Drop support for Ruby 1.8 (including ree)

- initiated by lack of 1.8 support on dependancy (Virtus 1.0)
Use Virtus 1.0
use provided Virtus Float types
use 1.9 hash syntax
use require_relative in specs
remove comments