Added grouped metrics feature

This pull request introduces three new API calls available to users who want to group or tag metrics monitoring a common task together:
```
    > folsom_metrics:tag_metric(Name, Tag).
    > folsom_metrics:get_metrics_value(Tag).
    > folsom_metrics:get_metrics_value(Tag, Type).
```

* I added unit tests to make sure the new features work as expected
* I modified the README file accordingly, how the new functions are to be used is described there.

Please review and comment, I'll make any correction needed for merging requested.

Thanks!