Adds time zone support for provider methods returning DateTime instance

[bishopb]

There are four DateTime returning methods: dateTime, dateTimeAD, dateTimeBetween, and dateTimeInterval, Two of these (dateTimeBetween and dateTimeInterval) set the timezone on the returned object, while the other two do not. This can lead to user land juggling timezones when comparing dates returned from the different generators.

This PR updates these four DateTime returning methods to set the time zone to result of date_default_timezone_get or the user-specified timezone given in the final argument to all methods. Test coverage updated to check that the default and custom time zones are handled.

[fzaninotto]

This is a BC break, because dates that were using date_default_timezone_get() now use UTC by default. Please consider using date_default_timezone_get() as the default, or I'll have to close the PR.

[bishopb]

@fzaninotto Ah, yes. Fixed and tests passing.

[fzaninotto]

Great! One thing is missing though: could you update the part of the Readme that documents the formatters you've modified to add the new parameter?

[bishopb]

@fzaninotto Done, but looking over the code again, I think the other DateTime methods should also take $timezone to be consistent. Specifically dateTimeThisCentury, dateTimeThisDecade, dateTimeThisYear, and dateTimeThisMonth.

What do you think?

[fzaninotto]

Thanks a lot!