Switch grade-school from tabs to spaces
Fix compiler warning for grade-test's add/3 spec

    school.exs:11: warning: string() type use is discouraged. For character lists, use char_list() type, for strings, String.t()
        school.exs:11: School (module)
Have grade-school test description match the exercise README

There was an inconsistency on whether or not getting a list of students
should return the names alphabetically or not. The README only specified
alphabetical order for the `sort/1` function, so rather than fix the
test, I just updated the description.
Merge pull request #28 from elasticdog/grade-school

Few minor fixes for the grade-school exercise