:traps setting (on by default)

This pull request features `:traps` setting to disable/enable signal handling from issue #756. Issue was reported by @michaelklishin, @rkh provided a nice API.

**API**

```ruby
class MyApp < Sinatra
  disable :traps
end

MyApp.run!
```

or:

```ruby
class MyApp < Sinatra
  # ...
end

MyApp.run! traps: false
```