Finish a TODO in rubocop.yml: ClassLength (lower to 100).

Use `class_eval` to define these methods to reduce class length:

`with_proxy`, `with_params`, `with_form`, `with_json`, `with_body`, `with_follow`.
Merge pull request #134 from JuanitoFatas/class-length

Finish a TODO in rubocop.yml: ClassLength (lower to 100).
Extract response status and provide some sugar.

Example:

    res = HTTP.get "http://example.com"
    fail "something wrong" unless res.status.ok?
Bump up docs coverage threshold