implemented keySetStringF
improve doxygen file

find more examples
fix file patterns
support for std::string
find tutorials
introduce elektraFormat

for usage outside from keySetStringF
tests: make more clear where string end+start
let error plugin also fail at kdbOpen

+ introduce README.md
improve plugin docu (part 1)
comment out not working examples
added ELEKTRA_ADD_WARNINGF and ELEKTRA_SET_ERRORF
Allow formatted errors

with ELEKTRA_SET_ERRORF and ELEKTRA_ADD_WARNINGF
old macros are still there

new macros were needed because C99 does not allow
no va-arguments (there is a ## trick in gcc, but its not C99)
and the format checking would be problematic ("%s" for simple string
needed)
improve checker + add kdb checking
always have error after warning
only use warnings during opening of plugins
warn at all problems in kdbOpen
document precondition
fix error plugin+add test case for it
update NEWS
Merge branch 'error'

fixes #12
Merge branch 'master' of github.com:ElektraInitiative/libelektra