Now callback functions get the URI of the request, this allows to use a
single resource for multiple different URIs.
The is_json flag is now gone for the to-string function, instead the
macro has an is_str flag. If set this automagically produces quotes
around the string for json output.
Now from-string functions can return an error-code, 0 for success, -1
for error.
Now there is a generic resource that can generate and parse
application/json as well as text/plain. It can be re-used, only the
from_string and to_string routines have to be written and the resource
properly set up. A new resource format is specified, see
GENERIC_RESOURCE in, e.g., examples/osd/pwm-example. This is now used in
all my examples, namely pwm-example, arduino-sketch, wallclock-time.
There was an off by one error for the month in time formatting (in
gmtime and localtime). And the leap-year computation was broken. Both
fixed now, so we get a correct date. For localtime we are still 2 hours
off because daylight saving isn't implemented yet.
Also renamed gmtime to utc.
