tiny improvements to timezone module: make list protected

[chillu]

Moved from http://open.silverstripe.org/ticket/5109, cc @sunnysideup

I just had a look at:

http://open.silverstripe.org/browser/modules/timezoneawareness/trunk/code/TimezoneField.php

I am wondering if we should have lists like timezones (and also countries and other generic lists) in a special folder in sapphire rather than invididual files. That makes it more likely that several classes can and will use the same list.

Also, here are some specific recommendations for http://open.silverstripe.org/browser/modules/timezoneawareness/trunk/code/TimezoneField.php

$timezone_list should be a static protected variable rather than a public one, using a get and set function for write/read access.

In the description, "over" should be "cover".