Additional checks to perform on "System Requirements" step before installation/upgrade

Some things popped up when Dmitry was installing In-Portal 5.2.0-B2 on beta server:

1.minimal version of PHP should be lowered to 5.2.0, since not all hosting companies have PHP 5.3 up and running on their server (despite speed improvements in existing PHP scripts)

2. we need to check for SPL (standard php library) extension presence (since we use it), which is enabled by default, however you can turn it off during PHP compilation time

3. check, that we can set both init_set('display_errors') and error_reporting settings, since when we can't do it any Fatal error that can popup won't be recorded in server log and won't be displayed at all; we of course disabled error reporting by default

4. check JPEG availability using function_exists('imagejpeg') and not by searching JPEG work inside GD supported library list

5. find and replace all places that use:
$count = $node->count(); // PHP > 5.3
with:
$count = count($node->children()); // PHP < 5.3

Add TODO comment where we specify to use 5.3+ version once we switch to it.