Refactoring in TextBlock SWT code and Implement Strikethrough/Strikecolor

Pulled out setting underline/underlineStyle/underlineColor & strikeout/strikeColor into methods. Correct me please if that is not useful or cleaner :)

Allows setting strikethrough and strikecolor options for a Shoes program. strikethrough and strikecolor are on the roadmap for this milestone and haven't been implemented yet.

RSpec tests for strikethrough and strikecolor.