[ASMapNode] Add custom pin annotation for static maps (#1890)

* Adds possibility to have custom annotation pins on static map. This resolves #1889.

* Removes wrong example for map annotations and adds some annotations to correct map example. #1889

* Static map node now uses specific property block to get annotation views.

* Changes self to strongSelf inside of the snapshotters completion block.

* MapNode: Adds statement in documentation.

* MapNode: Block for annotation view/image now returns UIImage and center offset is returned in inout param.

* MapNode and map example: Fixes from review.

* MapNode example: Gets image directly from custom annotation, without creating annotation view.

examples/ASMapNode/Sample/CustomMapAnnotation.m

@@ -0,0 +1,22 @@
+//
+//  CustomMapAnnotation.m
+//  ASDKMapTest
+//
+//  Copyright (c) 2014-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+//
+//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+//  FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+//  ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+//  CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+//
+
+#import "CustomMapAnnotation.h"
+
+@implementation CustomMapAnnotation
+
+@end