Update StringUtil.php to handle birthdates before 1970

[pumpkinball]

Fixes Issue 272 when returning Employee DateofBirthAsDate, now handles birth years before 1970

[CodeSmith32]

@pumpkinball
Thanks! This is a much needed fix! But there are a few issues with this PR:

1. The comma in the character group, line 39, is wrong. Timezones only start with + and -, not ,. Should just be [+-].

2. This PR may create a new issue: When the date does not contain a timezone offset, the code throws an exception. According to Xero's documentation, dates may take the form, '/Date(1439434356790)/' or '/Date(1419937200000+0000)/', but this PR does not handle the former case. If $matches[2] is empty, presumably you need fall back to '+0000' before creating the DateTimeZone, e.g., new \DateTimeZone($matches[2] ?: '+0000').

3. Please fix the broken return type as well, line 30. It says @return $this which was never correct even before this PR, and screws up intellisense. Should be @return \DateTime.

4. Not critical, but perhaps consider what happens if the pattern doesn't match. The previous version raises a warning but returns a zero date, 1970-01-01 00:00:00.000000. This version raises 3 warnings, and (once the timezone issue is fixed) will also return a zero date. Maybe the safest approach would be to raise a single, clearer warning, and return a zero date.