Chore/unit conversion

Unit conversion functionality plus an ordered map class to store unit conversion ratios by name.  This can be instantiated from a property list which will have to be in the format of an array of dictionaries.

    Array [ 
        Dictionary {
          @"ratio":    5.1
          @"name":  @"inches"
        }
    ]

The units are stored in a dictionary and the names of the units are stored in an ordered set container.  This allows the unit map to return units in alphabetical order, and still use the mutable dictionary to manage the association of units to names.

Unit ratios are based on a base unit and so the ratio of the base unit is always 1.  This limits the unit map from having more then one type of unit conversion (i.e. temperature and length).  To solve this I would want multiple view controllers presented in an easy to navigate interface of categories.  Each category will have it's own unit map, view controller and source property list.