examples, doc: Add backend timers example.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Update general information.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
pywrap: loaders: Add a new methods.

* Add error checking for TIFF Load and Save methods.

* Add new Save methods into the loaders submodule.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: loaders_python: Update.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: PNG: Change the ReadPNG() and OpenPNG

The ReadPNG() now expects the PNG header too and the OpenPNG() does
rewind the file after the signature is checked.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
container: ZIP: Add support for loading PNG.

We need to add more general loaders here, but that is
not possible without small changed in general loaders
structure, so add a bit hacky support for PNG for now.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>