Extract out a JS service for calculating where to centre the map when given a set of coordinates.

Also removing the hardcoded default latitude/longitude from open_street_map directive because it's probably not very likely that it will be needed.

spec/javascripts/unit/darkswarm/services/map_centre_calculator_spec.js.coffee

@@ -0,0 +1,70 @@
+describe 'MapCentreCalculator service', ->
+  MapCentreCalculator = null
+  defaultLongitude = null
+  defaultLatitude = null
+
+  beforeEach ->
+    module 'Darkswarm'
+    defaultLongitude = -6
+    defaultLatitude = 53
+
+    inject (_MapCentreCalculator_)->
+      MapCentreCalculator = _MapCentreCalculator_
+
+  describe "calculate_latitude", ->
+    it "calculates the center latitude", ->
+      coordinates = [
+        { latitude: 53, longitude: defaultLongitude },
+        { latitude: 54, longitude: defaultLongitude }
+      ]
+
+      expect(MapCentreCalculator.calculate_latitude(coordinates)).toEqual 53.5
+
+  describe "calculate_longitude", ->
+    it "calculates the center longitude", ->
+      coordinates = [
+        { latitude: defaultLatitude, longitude: -6 },
+        { latitude: defaultLatitude, longitude: -7 }
+      ]
+
+      expect(MapCentreCalculator.calculate_longitude(coordinates)).toEqual -6.5
+
+  describe "_calculate", ->
+    it "calculates the average angle correctly when given a single angle", ->
+      coordinates = [
+        { latitude: defaultLatitude, longitude: -7 }
+      ]
+
+      expect(MapCentreCalculator._calculate("longitude", coordinates)).toEqual -7
+
+    it "calculates the centre correctly when given a set of positive angles", ->
+      coordinates = [
+        { latitude: 53, longitude: defaultLongitude },
+        { latitude: 54, longitude: defaultLongitude }
+      ]
+
+      expect(MapCentreCalculator._calculate("latitude", coordinates)).toEqual 53.5
+
+    it "calculates the centre correctly when given a set of negative angles", ->
+      coordinates = [
+        { latitude: defaultLatitude, longitude: -6 },
+        { latitude: defaultLatitude, longitude: -7 }
+      ]
+
+      expect(MapCentreCalculator._calculate("longitude", coordinates)).toEqual -6.5
+
+    it "calculates the centre correctly when given a mixture of positive and negative angles and the centre is positive", ->
+      coordinates = [
+        { latitude: defaultLatitude, longitude: 7 },
+        { latitude: defaultLatitude, longitude: -4 }
+      ]
+
+      expect(MapCentreCalculator._calculate("longitude", coordinates)).toEqual 1.5
+
+    it "calculates the centre correctly when given a mixture of positive and negative angles and the centre is negative", ->
+      coordinates = [
+        { latitude: defaultLatitude, longitude: 4 },
+        { latitude: defaultLatitude, longitude: -7 }
+      ]
+
+      expect(MapCentreCalculator._calculate("longitude", coordinates)).toEqual -1.5