add shareActivityType for iOS to specify different data for (Facebook, Twitter, Email, Clipboard, etc)

- used by https://github.com/banterchat/legit/pull/903

Allows JS clients to provide different data to share depending on the share destination (e.g. Facebook, Twitter, Email, Text Message, clipboard).

Currently only works on iOS, we'll need to revisit it for Android.

For example a client could implement a template system and then pass the following datastructure to `socialshare.shareForActivityType(data)`
```
{
    "PostToFacebook": {
        "msg": "Check out my rankings for {{=it.collection.name}}",
        "link": "{{=it.link}}"
    },
    "PostToTwitter": {
        "msg": "Check out my rankings for {{=it.collection.name}} on @legitapp",
        "link": "{{=it.link}}"
    },
    "Mail": {
        "subject": "Are we compatible? Check out my rankings for \"{{=it.collection.name}}\"",
        "msg": "<html><body>I just completed <a href=it.\"{{=it.link}}\">{{=it.collection.name}}</a> on Legit - check out my rankings and see if we agree.<br/><br/><a href=\"{{=it.link}}\">View My Rankings</a></body></html>",
        "link": "{{=it.link}}"
    },
    "CopyToPasteboard" : {
        "link": "{{=it.link}}"
    },
    "Message" : {
        "msg": "Are we compatible? Check out my rankings for \"{{it.collection.name}}\" on Legit: ",
        "link": "{{=it.link}}"
    },
    "default": {
        "link": "{{=it.link}}"
    }
}
```

- uses https://github.com/rdougan/RDActivityViewController to provide a UIActivityViewController that allows an array of data depending on the activity type being shared