Exception#set_backtrace

Specs added:

Exception#set_backtrace
 - allows the user to set the backtrace to string
 - allows the user to set the backtrace to nil
 - raises TypeError when argument is not array
 - raises TypeError when argument contains non String element
Fixed titles
make title more explicit
Exception#set_backtrace also accepts empty array
Merge pull request #209 from krekoten/exception_set_backtrace

Exception#set_backtrace