Initial implementation of SortedList.

Supports:
Constructor with custom key and less-than functions
add(), shift() and pop() between O(log16 N) and O(log32 N)
ES6 iteration at O(N)
first() and last() at O(1)

Many other functions are either absent or will crash when run. See my pull request for more information.

src/Immutable.js

@@ -8,6 +8,7 @@
 import { Seq } from './Seq';
 import { OrderedMap } from './OrderedMap';
 import { List } from './List';
+import { SortedList, isSortedList } from './SortedList';
 import { Map } from './Map';
 import { Stack } from './Stack';
 import { OrderedSet } from './OrderedSet';
@@ -64,6 +65,7 @@ export default {
   Map: Map,
   OrderedMap: OrderedMap,
   List: List,
+  SortedList: SortedList,
   Stack: Stack,
   Set: Set,
   OrderedSet: OrderedSet,
@@ -85,6 +87,7 @@ export default {
   isValueObject: isValueObject,
   isSeq: isSeq,
   isList: isList,
+  isSortedList: isSortedList,
   isMap: isMap,
   isOrderedMap: isOrderedMap,
   isStack: isStack,
@@ -119,6 +122,7 @@ export {
   Map,
   OrderedMap,
   List,
+  SortedList,
   Stack,
   Set,
   OrderedSet,