Clean up all the #include sections

This commit:

- Sorts the #include directives: The #includes of the ""-form come
  first, those of the <>-form come later (this helps checking if any
  include file is not self-sufficient); in each of the two groups, the
  names of the headers or source files are sorted alphabetically (this
  eases searching for a specific #include and avoids duplicates).

- Uses the ""-form when including Boost files; using the <>-form is a
  relic of the past. This was even discussed on the developers list many
  years ago and led to core issue 370. Note that some parts of
  DynamicBitset were already using the ""-form and that has never caused
  any problem, AFAIK.

- Removes some comments attached to the directives themselves which
  seemed pretty useless and prone to get out of sync.

example/example2.cpp

@@ -10,9 +10,8 @@
 //  bits(2) = 10
 //  bits(3) = 11
 
-
+#include "boost/dynamic_bitset.hpp"
 #include <iostream>
-#include <boost/dynamic_bitset.hpp>
 
 int main()
 {