Fixes #24
Add multithread test (failing)
Fixes #22
Let API.run accept debug option
Changed unhandled_exception_hook behavior

- Pass request object as second parameter
- Don't call it when Server.debug is true
New thread-local API

- `SafeGlobal` gone in favor of `SwappableDict` and `ThreadLocalDict`
- Swapping API instead of stack API
- `thread_local` context manager takes care of cleaning up (fixes #26)
Update docs for new thread-locals
Release v0.4.2