Create one enviromental check to recommend PHP >= 5.3.3 on install/upgrade

While PHP 5.2.8 is the hard (required) min version to run Moodle 2.0, I think it could be interesting to add one new environmental test to be executed on each install / upgrade:

• Create it as custom and optional check.
• Make it look for PHP 5.3.3
• If the test pass, remain hidden.
• If the test fails, inform about the benefits (memory/speed/security) of using PHP 5.3
• Make the test available also in 19_STABLE (but apply it only to 2.0 version)

I think it can be interesting to have that being informed continuously to admins. Differences are huge enough.

Ciao