translate Paths to valid dot-separated MongoDB collection names (Fixes #139)

Shouldn't this search proceed by "deepest prefix"?

i.e. if we have the mappings:

```
/foo/bar/buz -> mongodb
/foo/boo -> riak
```

and then look for a backend for '/foo/bar/', then we should get back 'mongodb' with a path remainder of '/buz'. I don't think the existing logic will do that.