Add #displace

Here's a first stab at adding `#displace`. It doesn't work quite right yet...but it also doesn't work quite right in Shoes 3. I added [samples/good-displace.rb](https://github.com/shoes/shoes4/blob/c1a142a0d6d34610b07f93f4d833065a86b52dde/samples/good-displace.rb) for playing with `#displace`.

Shoes 3:

![image](https://f.cloud.github.com/assets/908/2073411/781c13d6-8d5a-11e3-8a27-d84b53cabd9e.png)

Shoes 4:

![image](https://f.cloud.github.com/assets/908/2073413/8973ae82-8d5a-11e3-98a1-2ac8ec85b269.png)

Already you can see that there are layout issues, but these are not related to `#displace`. Check out after clicking each of the `displace` buttons:

Shoes 3:

![image](https://f.cloud.github.com/assets/908/2073417/baf6307e-8d5a-11e3-9d0c-cbd141b526a2.png)


Shoes 4: 

![image](https://f.cloud.github.com/assets/908/2073419/c8ce6ca2-8d5a-11e3-94b0-a5f90acafea3.png)

And now after clicking each of the `replace`