Support for reverse search terms

I've created [hamster-trac-syncr](https://github.com/alexhayes/hamster-trac-syncr) which provides the ability to sync Hamster activity to [Trac](http://trac.edgewall.org/).

In doing this I needed to add the ability to reverse the search terms for `GetFacts` and thus in the [installation instructions](https://github.com/alexhayes/hamster-trac-syncr#requirements) I've noted that users need to use [my fork](https://github.com/alexhayes/hamster) of hamster however, obviously, it would be much better if users didn't have to do this and this functionality existed in upstream.

This pull request adds the ability to reverse the search terms however I'd be happy to modify hamster-trac-syncr if you know of a better way to do a reverse match? Currently I do the following (as taken from [hamstertracsyncr/\_\_init\_\_.py#L52](https://github.com/alexhayes/hamster-trac-syncr/blob/master/hamstertracsyncr/__init__.py#L52))

```python
for fact in storage.get_facts(start, end, TAG_SEARCH, True):
    ...
```

Thoughts?