Date/time big fixes

As discussed here: https://github.com/concrete5/concrete5-5.7.0/pull/854#issuecomment-54531902, lets:
- deprecate getSystemDateTime
- deprecate getLocalDateTime
- deprecate dateTimeFormatLocal
- add a method to convert any date/time representation to the sql fomat (`toDB`)
- add a method that retrieves the current date/time or, if present, the *custom request date time* (I called it `getOverridableNow` so that it's clear that the current date time can be overridden by the custom request date/time.

There are still some date/time issues (mainly related to localization and timezones), but if and once this PR gets merged everything will be easier.