Create isNumber function to test for numbers
Add 'isQty' function to check for quantity instances
Assert valid initialization value type
Keep initializing value untrimmed
Throw error when parsing blank strings

This is a breaking change to match Ruby-units behavior. Previously,
blank strings was instanciated as unitless quantities with 1 as scalar.
Accept numbers as initialization values

Passing number as initialization value create a uniless quantity with
passed number as scalar. This change allows to match with Ruby-units
behavior.
Update README