Edits to Panorama stitching example

- Minor wording edits at the top of this section.
- Change uppercasing from "SAMple" to "SAmple" on (RANSAC)
- Remove errant comma
- Move `SimilarityTransform` import down to appropriate block of code
- Reformat alpha-channel function for readability
Merge pull request #48 from tonysyu/edits/panorama

Edits to Panorama stitching example