Add system requirements check

[joshas]

Before installation installer should check if user has correct PHP version and required extensions, MySQL version and directory permissions.

[claviska]

You can find the check for PHP version and extensions in runtime.php. I kept them there in case people try to migrate without running the installer and the checks are negligible in terms of performance. However, the installer does check permissions by performing a write/read test on all required folders. This only happens initially because it would be a performance hit. :)

[joshas]

I initially tried running Postleaf on older PHP version and it would have saved some time if installer warned me about PHP version being too old. Just a nice thing that some other CMSes have.

[claviska]

Whoops, the check in runtime.php was for 5.4 when it actually requires 5.5+. All fixed for beta2 :)