`with_backtrace` context helper method

This method takes a backtrace and block.  It then runs the block and
takes any results generated by the block and replaces their backtrace
with the given backtrace.

This allows 3rd-party tools to write their own custom assertions
and have them abstract the 3rd-party backtraces that any results
would have.

An example usage would be:

```ruby
def assert_some_custom_behavior(*args)
  with_backtrace(caller) do
    # asset some things (generate results)
  end
end
```

**Note**: this _does not_ affect error results.  These results need
to have "pure" backtraces to assist in debugging (error result
backtraces are never modified in Assert).  This _will_, however,
affect skip and halt-on-fail results: `with_backtrace` rescues the
exceptions raised in those scenarios and sets the exception's
backtrace to the given backtrace and then re-raises.  This way you
get both the halt behavior of the result and the backtrace abstraction
of `with_exception`.
Merge pull request #133 from redding/kr-failfrom

`with_backtrace` context helper method