Implement ELPM_2, ELPM_3

`ELPM` (Extended Load Program Memory) was implemented in its first form, which loads `Z` into `R0`, but not its second or third forms, which loads `Z` or `Z++` into `R[d]`. I implemented these functions.

![ELPM reference](https://f.cloud.github.com/assets/118362/1223015/75c4e096-270b-11e3-8d74-432497883311.png)