Added the option to create mocks using a deeply nested set of folders, r...

Added the option to create mocks using a deep(er) nested set of folders, rather than just directories with a lot of files in them.  This helps organize a large API by category, so that all requests to `/test` end up in the `test/` directory, while calls to `/test/1` end up in `test/1/`, etc.

* The changes allow a single boolean to be passed when configuring the mockserver, which turns a file like `test--a=b_GET_extra.mock` into `test/GET_extra--a=b.mock`.
* There are new tests for the new code.
* I updated the README.
* The new functionality should not affect existing functionality.
* I also did my best to match the existing code style as close as I could.