Changed file type detection to reflect new lowercase default behavior

When creating a new todo/ directory structure from the Android app last
week I noticed new todo files are now (have always been?) created all
lowercase.

Therefore I made a trivial change to ftdetect/todo.vim to make the plugin
recognize both Todo.txt as well as todo.txt and switch to todo mode
accordingly.
Merge pull request #2 from azmd/caseregex

Changed file type detection to reflect new lowercase default behavior