Fix the install target

This commit fixes two problems related to cake install.

Firstly it was assumed that the prefix directory contains a
subdirectory named bin. When it didn't exist the script failed when
creating a symlink to docco. The problem is fixed by creating the bin
subdirectory.

Secondly an attempt to install (i.e. cp -rf) a directory named lib was
made. However the project doesn't contain such subdirectory. It was
removed from the list of directories to copy.
Fix the -m/--marked option

The argument to -m/--marked is a JSON file name containing additional
optins that should be passed to Marked. The problem was that the
default value of the command line option, as specified in the defaults
objects, was set to an empty object which definitely isn't a valid file
name. The result was an attempt to fs.readFileSync({}) which obviously
raised an exception.

This commit fixes the problem by setting the default value of
-m/--marked to null.
Merge pull request #290 from grn/master

Cakefile and CLI bug fixes