Adding new unit requires boilerplate

To create new unit it is required to provide `Units` type and an `ComplexUnit` instance. `ComplexUnit` is used to instantiate Units instances (see [link](https://github.com/ppiotrow/physical-units/blob/master/src/main/scala/com/ppiotrow/units/Units.scala#L34))

``` scala
type Speed = Units[Meter :: HNil, Second :: HNil]
object Speed extends ComplexUnit[Meter :: HNil, Second :: HNil]
```
