Tag structure

Currently, tags are defined as follows:
```
type Tag struct {
	Key   string
	Value string
}
```

This seems somehwhat inconsistent with the [spec](http://sentry.readthedocs.org/en/latest/developer/client/index.html#building-the-json-packet), which provides the following examples:

```
{
    "tags": {
        "ios_version": "4.0",
        "context": "production"
    }
}
```

```
{
    "tags": [
        ["ios_version", "4.0"],
        ["context", "production"]
    ]
}
```

Considering the first example only, it seems that `Tags` could be replaced with `map[string]string`. This does not hold true for the second example though :-1: