Support /- and nested properties

[marcbachmann]

Adds set /- support, adds nested property setter:

// set nested properties
assert.equal(jsonpointer.set(obj, "/f/g/h/i", 6), undefined);
assert.equal(jsonpointer.get(obj, "/f/g/h/i"), 6);

// set an array
assert.equal(jsonpointer.set(obj, "/f/g/h/foo/-", 'test'), undefined);
arr = jsonpointer.get(obj, "/f/g/h/foo")
assert(Array.isArray(arr), 'set /- creates an array.');
assert.equal(arr[0], 'test');

Requested in #14

[marcbachmann]

This is an improvement over the current implementation where we've just returned null.
At least setting values works now.
There are no downsides as I can see. However, I will do a major release. I don't want to break other libraries.