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

package.json

@@ -9,6 +9,7 @@
       "amazon-fireos",
       "ubuntu",
       "ios",
+      "osx",
       "wp7",
       "wp8",
       "windows8",
@@ -31,6 +32,7 @@
     "cordova-amazon-fireos",
     "cordova-ubuntu",
     "cordova-ios",
+    "cordova-osx",
     "cordova-wp7",
     "cordova-wp8",
     "cordova-windows8",