Ability to check whether a version belongs to certain series

This can be useful, I can then implement it into th BZ plugin for better checking of versions. Basically you can check whether certain version belongs to another version's series (like 5.2.5.2 belongs to 5.2 series and not to 5.1 or 5.3). This is done by cutting the specific version's fields to align with the series and then it is simply compared. The interface is very simple and actually quite straightforward:

```python
specific_version.is_in_series(series_version)
# But we got a nice DSL shortcut here
specific_version in series_version
```

It works both with Version classes and strings are automatically converted to them.