added a method 'do_savefirst' for saving the first frame of every animation/image

Helpful for indexing a DRS/SLP from AOE HD or an expansion pack that doesn't mention the item ids.

A lower range and higher range are given as params, this spits out id1.png - id10.png

![screen shot 2014-05-24 at 00 48 01](https://cloud.githubusercontent.com/assets/1926968/3073012/b76d65ba-e2d4-11e3-8fa0-16abc1798822.png)

I hope this is something that interests you, feel free to decline!!