Extract response status and provide some sugar.

Changes overview

- Deprecates `HTTP::Response::STATUS_CODES` (will be completely removed in 1.0.0)
- Deprecates `HTTP::Response::SYMBOL_TO_STATUS_CODE` (will be completely removed in 1.0.0)
- Deprecates `HTTP::Response#status_code` (will be completely removed in 1.0.0)
- Wraps `Fixnum` status code with `HTTP::Response::Status` delegation class
- Makes `HTTP::Response#code` a proxy to `status.code`
- Makes `HTTP::Response#reason` a proxy to `status.reason` 

Example:

``` ruby
res = HTTP.get "http://example.com" 
fail "something wrong" unless res.status.ok?
```