Fix tag merging implementation to be on-spec

[taylortrimble]

From the spec:

Tags are key/value pairs that describe an event. They should be configurable in the > following contexts:

• Environment (client-level)
• Thread (block-level)
• Event (as part of capture)

Each of these should inherit its parent. So for example, if you configure your client as so:

client = Client(..., {
    'tags': {'foo': 'bar'},
})

And then you capture an event:

client.captureMessage('test', {
    'tags': {'foo': 'baz'},
})

The client should send the following upstream for tags:

{
    "tags": [
        ["foo", "bar"],
        ["foo", "baz"]
    ],
}

What does this mean for us? We should rethink how we merge tags, and allow duplicate "keys" by making packet.Tags something like this:

type Tag struct {
    Key   string,
    Value string
}

type Packet struct {
    ...
    Tags       []Tag      `json:"tags,omitempty"`
    ...
}

[titanous]

Yeah, this makes sense.