Typhoeus

experimental Typhoeus support. Default Net::HTTP behavior should be unchanged.

I haven't yet used this in my project yet -- perhaps hold off on merging until I've done some more sanity checks.

Let me know if you think this is a good direction. If so then I will add more tests.

----
see discussion in #60 

### to decide / waiting on
- [x] see what happens with https://github.com/typhoeus/typhoeus/issues/373
- [x] see what happens with https://github.com/typhoeus/typhoeus/issues/393
- [x] use HTTP adapters or just switch to only typhoeus?
- [x] continue to support "safe" redirects?
 
### todo
- [ ] remove plugable HTTP adapter system
- [ ] somehow switch to http mocking system which supports typhoeus. fakeweb only supports Net::HTTP requests. would be nice to have some sort of elegant way to not have to wrap every single example in the test suite with VCR