Exception#set_backtrace

Specs fixed:

Exception#set_backtrace
- accepts an Array of Strings
- allows the user to set the backtrace from a rescued exception
- accepts an empty Array
- accepts a String
- accepts nil
- raises a TypeError when when passed a Symbol
- raises a TypeError when the Array contains a Symbol
Add w_backtrace to attributes
Use is_kind_of
Merge pull request #683 from krekoten/exception_set_backtrace

Exception#set_backtrace