Add recursive directory (& contents) removal to io

In order to achieve this, I performed some restructuring:

1. Make the io implementation modules actually inherit from _base, but
statically (should this be a documented pattern in the coding style wiki
page?)

2. Implement readDir and stat methods in staticNodeJS module only (it’s
not obvious to me how to implement them in REST).

3. Implement rmDirRecursive as an abstract method. At the moment it will
only work in Node, because of the missing methods in REST (see 2 above).
Merge pull request #20 from rrthomas/master

Add recursive directory (& contents) removal to io
This opens #22, #23 and #24