Add method for changing the issuing organization of a program

Fixes #250 

![see](http://i.cloudup.com/hudRU6CeLQ.png)

This introduces the necessary changes for being able to move a program to another organization:

* Change the `program.issuer`
* Remove it from the `oldIssuer.programs` array
* Add it to the `newIssuer.programs` array.

@cmcavoy to review.