React to form changes, compute unit price and display accurate values.

- Add method `processUnitPrice` which is responsible for computing the right unit price, that depends on `price`, `variant_unit_scale`, `variant_unit`, `unit_value` and `variant_unit_name`
 - Watch the needed model to compute the unit price: `product.price` and  `product.variant_unit_name`
 - Add dependencies : UnitPrices and localizeCurrencyFilter
 - Add currencyconfig to spec, as it's needed by localizeCurrencyFilter
 - Put `'ng-controller' => 'unitsCtrl'` to the relevant node.
 - Add new ng-model, as it's needed to watch it in order to compute unit price : `product.price`
 - Finally display the needed information: `product.unit_price_value` and `product.unit_price_unit`

spec/javascripts/unit/admin/products/units_controller_spec.js.coffee

@@ -2,11 +2,17 @@ describe "unitsCtrl", ->
   ctrl = null
   scope = null
   product = null
+  currencyconfig =
+    symbol: "$"
+    symbol_position: "before"
+    currency: "D"
+    hide_cents: "false"
 
   beforeEach ->
     module('admin.products')
     module ($provide)->
       $provide.value "availableUnits", "g,kg,T,mL,L,kL"
+      $provide.value "currencyConfig", currencyconfig
       null
     inject ($rootScope, $controller, VariantUnitManager) ->
       scope = $rootScope