CB-11955 Added Initial OSX platform support

- Added src/osx plugin folder structure.
- Added OSX platform to plugin.xml and package.json files.
- Added _system target functionality for OSX.
- Modified README.md to include information about OSX support.
- Disabled 'open method' jasmine tests for OSX.

 This closes #190

src/osx/CDVInAppBrowser.h

@@ -0,0 +1,30 @@
+/*
+ Licensed to the Apache Software Foundation (ASF) under one
+ or more contributor license agreements.  See the NOTICE file
+ distributed with this work for additional information
+ regarding copyright ownership.  The ASF licenses this file
+ to you under the Apache License, Version 2.0 (the
+ "License"); you may not use this file except in compliance
+ with the License.  You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing,
+ software distributed under the License is distributed on an
+ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ KIND, either express or implied.  See the License for the
+ specific language governing permissions and limitations
+ under the License.
+ */
+
+#import <Cordova/CDVPlugin.h>
+
+@interface CDVInAppBrowser : CDVPlugin {
+}
+
+@property (nonatomic, copy) NSString* callbackId;
+
+- (void)open:(CDVInvokedUrlCommand*)command;
+
+@end
+