Fix multiline sorting with force-alphabetical-sort

Raised in issue 364.

The separate code for --force-alphabetical-sort has been removed and
a boolean flag added to handle sorting with or without case in the
_module_key function (which handles the sorting).

Note this commit alters these 2 tests:
test_alphabetic_sorting_no_newlines, test_alphabetic_sorting
This results in the lone imports being moved to the top of the file.
I think this is better as it means the force_alphabetical_sort tests
now behave similarly to the other tests in the file.