Adds support for tagging logs emited via python logging module

I didn't find any straightforward way to tag logs within sentry context using traditional python logging module. This pull request adds support for tagging logs using python logging module. I think this functionality can be added within raven core and not just to `raven.contrib.django.handlers`. I will be happy to modify this PR if required.

```
>>> import logging
>>> logger = logging.getLogger(__name__)
>>> logger.debug("log message that will be tagged within sentry context", extra={"tags":{"key1":"val1", "key2":"val2"}})
```