Added the choose function to the So data type section
Improvements to the case-study.

+ Adds section headings.
+ Improves formatting of language example.
Made screen have a sans-serif font as default.
Updated to new idrislang.sty
Added new idrislang.sty
Updates.
Merge branch 'new/idrislang.sty' of github.com:jfdm/idris-tutorial into new/idrislang.sty
Merge pull request #52 from jfdm/new/idrislang.sty

Update to latest idrislang.sty
Merge pull request #48 from gavlegoat/master

Added the choose function to the So data type section
Merge pull request #49 from jfdm/fix/interp-anon-funcs

Improvements to the case-study.
Merge pull request #50 from jfdm/fix/screen-with-sf-font

Made screen have a sans-serif font as default.