Added cr_skip_test to skip a test during runtime. Also merged the old
disable system with the new one. All skipped tests are printed by
normal_log_post_suite(), so there is no need for a logger function for
runtime skipepd tests.
Fixes#131.
- Default assertion messages will now print the expession and the content of both strings.
- String assertions will not crash, if the actual string is NULL.
(cr_assert_not_null(actual) is called before the cr_assert_str* assertion)
