Cakefile and CLI bug fixes

I fixed three bugs:

* `Cakefile` no longer assumes that the `--prefix` directory contains `bin` subdirectory. It attempts to create it which makes it possible to install to a non-existing directory (e.g. `/tmp/docco`)
* `Cakefile` doesn't try to install the `lib` which doesn't exist
* the default value of `-m`/`--marked` is set to `null` instead of an object

Without these fixes I wasn't able to build and run docco.

Keep up the good work!