code optimization

- changes switch-case to if/else
- defining helper functions before invoking them
adding test cases for changing Column.defaults.sortType to "toggle"

3 new test cases when setting the sortType to toggle - click on it
once, twice and 3-times
documenting Column.defaults.sortType

documenting the behaviour of setting ```sortType: "toggle"```
Merge branch 'feature/sortType' of git://github.com/morficus/backgrid into morficus-feature/sortType