Add support for checking minimum PHP version in XML update stream

This introduces a <php_minimum> tag which should indicate the minimum supported PHP version of an extension
Optimize version_compare check, thanks @dongilbert
Add message if an update is available but unsupported
Restructure PHP version check
Merge remote-tracking branch 'joomla/staging' into phpUpdateLib
[CODE STYLE] Corrected whitespace
Oops
Merge branch 'staging' into phpUpdateLib
Data object is a stdClass instance
Updating installation lang files: ar-AA, bg-BG, sw-KE, tr-TR
Add postinstall message informing of PHP version change in 3.3. Thanks
Michael Babker
Merge pull request #2802 from mbabker/phpUpdateLib

Add support for checking minimum PHP version in XML update stream
Missing lang string from #2802
Merge pull request #2883 from nonumber/patch-37

[CODE STYLE] Corrected whitespace
Updating installation lang files: cs-CZ, da-DK, he-IL, id-ID, it-IT,
ja-JP, nb-NO and id-ID credits
Merge remote-tracking branch 'origin/staging' into staging
[CODE STYLE] Several issues in com_categories and JFormFieldHelpsite
Fix #2928, #2929, #2930, #2931, #2932, #2933, #2934, #2935, #2936, #2937, #2938, #2939, #2940, #2941, #2942