remove the comparable interface
Merge pull request #18 from otaviojava/remove_comparable_interface

remove the comparable interface
Merge pull request #17 from otaviojava/refactor_quantity_interface

semantic correctness
UNITSOFMEASUREMENT-54: split funcionality in AbstractQuantity 
Improved JavaDoc
Task-Url: https://java.net/jira/browse/UNITSOFMEASUREMENT-54
using generics in factory method