Include stddef.h in utest.h

The NULL constant is used but never defined. This lead to errors when
compiling tests with ut_assert_[not_]null() but no include that defined
NULL.
Improve Makefile.am a bit
Simplify suite_run()
Only pass timer to _forked()

The complete result struct is not neccessary.
Use named struct for test
Pass test to functions directly instead of index

This should be less error prone as index into an array is not nice.
Enable turning forking off
Add disabling forking to _run_all_tests()
Add cmdline option for forking
Update documentation
Color hr properly

They may be used as footnote delimiters.
Write overview of the whole library
Bump version number