API: POST tag data into series/revisions

Tags are data pairs (key, value) which are attached to a specific series/revision entry point. This can be useful to attach some extra data into a series/revision.

REST entry point:

```
POST /api/1.0/series/100/revisions/2/tags/
```

Possible JSON object

```
{
"test_result" : "success",
}
```

or

```
{
"branch_merge": "master-next"
}
```

One of the uses of this new feature would to solve the following:
- https://bugzilla.yoctoproject.org/show_bug.cgi?id=7658

It is important to notice that there is no restriction for either tags's keys or values in terms of format; tag is just a way to attach project specific data to a series.

V2: we may limit the number of tags that can be attached to a series