Fixing layout of buttons in Mission View Controller on iPhone 4 and 4S

Currently the Accept/Start button in Mission View Controller appears halfway off the bottom of the screen on iPhone 4 and iPhone 4S devices, and the Skip button is partially covered by the image view and scrolling text view.

![screen shot 2013-07-05 at 00 16 53](https://f.cloud.github.com/assets/125896/751517/eb5344e0-e4ff-11e2-953b-baa21863bf00.png)

Changed the button to have a constraint that links it to the bottom of the screen.

Changed the ImageView and ScrollTextView so that they don't have a set size and instead base their size on the distance from the top and bottom of the screen.