updated TODO
merged TODO and TODO.rst from branch olf/skip
added method skip for conditionally skipping some tests

+ added SKIP variables
+ added SKIP_IF()
+ check number of positional parameters
+ add flag errexit to data test scripts
+ renamed SKIP_IF() to SKIP()
+ rewrote tests for SKIP()
+ added __skipped()
[doc] prove libtest with different shells
removed shebang from all test scripts
use right $SHELL for data test scripts
ensure removing temporary files
add shell option -e to every data test script
updated TODO
updates README
updated TODO
updated Changelog