Consider refactoring how we set mobile context

[jonalmeida]

When we have a determined/unchanging schema for how the context 'co' is going to be set, we should consider refactoring our 0.1 implementation to be configurable or generic. Right now, it feels a bit hard coded and ugly to read.

Early thoughts:

• SnowplowPayload should be an object which always has schema with data configurable as an array or object.
  • This would let us have nested payloads (see example payload below)
• [x] Fix inheritance from NSDictionary
• [x] Add inits with default schema
• [x] Solve how to make data optionally a dictionary or array ( EDIT: Use type id and check with [myObject isKindOfClass:[NSDictionary class]] )

{
    schema: 'iglu:com.snowplowanalytics.snowplow/payload_data/jsonschema/1-0-0',
    data: [{
        eid: 'BBBAEB98-B556-43E6-8A07-80B6FD093C2E',
        vp: '320x480',
        p: 'mob',
        co: {
            schema: 'iglu:com.snowplowanalytics.snowplow/contexts/jsonschema/1-0-0',
            data: [{
                schema: 'iglu:com.snowplowanalytics.snowplow/mobile_context/jsonschema/1-0-0',
                data: {
                    osType: '...',
                    osVersion: '...',
                    deviceVendor: '...',
                    deviceModel: '...',
                    carrier: '...'
                }
            }]
        },
        refr: 'myreferrer',
        url: 'http://foo.com',
        dtm: 1402062281979.792,
        tv: 'ios-0.1',
        tna: 'myname',
        lang: 'en',
        page: 'This is my foo',
        e: 'pv',
        aid: 'foo',
        res: '320x480'
    }]
}

[jbeemster]

This will be solved with a SelfDescribingJson class implementation.