Offers - Issue 405 - Section A

https://github.com/oscurrency/oscurrency/issues/405

Includes new:
+ Stylesheet
+ View - full view of `offer`
+ Helper method `horizontal_formatted_offer_categories`

Changes
+ Some custom styling was needed, prefer not to use in-line styling, new style-sheet added
+ Smaller text based categories description on top
+ Image to left, content to right and markdown on bottom
+ Separation created too much white space, attempted to reduce with spacing
+ A small border to separate the image form the text in right