only run substr() on strings in Stacktrace.php

can only do substr() on strings, so let's not try otherwise. (Was getting an issue where with a Guzzle exception the $value was showing up as a resource. It is working with this change)
Merge pull request #150 from thedamon/patch-1

only run substr() on strings in Stacktrace.php
add is_numeric check to substr()

add is_numeric since those can be implicitly converted to strings
Merge pull request #152 from thedamon/patch-2

add is_numeric check to substr()
Mark test as incomplete
Support chained exceptions in PHP 5.3+
Add test for chained exception
Dont label exception interfaces with default value