Allow setting the NSUserDefaults name for iOS implementation.

[kspearrin]

Please take a moment to fill out the following (change to preview to check or place x in []):

Fixes #309.

Changes Proposed in this pull request:

• The usage of app groups that share NSUserDefaults is not possible with the current implementation. This PR adds that ability to set a static DefaultsName so that it can be shared within an app group. See https://developer.xamarin.com/guides/ios/watch/working-with/parent-app/#Shared_Storage

This fixes/implements:

• [] Bug
• [x] Feature Request

Which plugin does this impact:

• [] Battery
• [] Connectivity
• [] Contacts
• [] DeviceInfo
• [] ExternalMaps
• [] Geolocator
• [] Media
• [] Permissions
• [x] Settings
• [] Text To Speech
• [] Vibrate
• Other:

[jamesmontemagno]

I just moved this repo over to http://github.com/jamesmontemagno/SettingsPlugin, could you send a PR down from there into a dev branch

[jamesmontemagno]

Implemented this recently in the setting plugin for all platforms.