Provide a way to mock a whole dir or tree.

[toddr]

We have a problem that one of the common algorithms this code does not support is when we open a temp file next to where we'll be putting the real file, write to it, then rename it into place as the real file. T:MF doesn't have a way to mock this sort of behavior.

Idea have been thrown out:

1. T::MF->dir(...) declares that anything created in that path is to be mocked.
2. T::MF->tree(...) declares a whole tree to be mocked.
3. T::MF->whole_dir(...)

These break the model of how we've been doing mocked files up to now. Maybe there's a way to make #115 pull off this need?