linked_list.{c,h}: Got rid of duplicated doc.
Encapsulation: Hides data structure from users.
Added support for version checking.

This way, you can force your application to be compiled only with a
specific version or a group of versions of this library.
API: Made function names shorter.
Restricts return values.
Reorder .c to match .h.
API allows freeing values directly.
Updated TODO with further development plans.
Improved documentation organization.
README.rst: Fixes tabulation.
README.rst: Provides example code.
test.c: Restricts type for counters.
README.rst: Improves documentation.