Add Response#parsed feature

Access to parsed body was nuked by merging #39.
This patch brings it back (feature, not the API).

Example:

    require 'json'

    res = HTTP.get('https://api.github.com/repos/tarcieri/http')
    res.parsed['description']
    # => "The HTTP Gem: a simple Ruby DSL for making HTTP requests"
Fix mimetype method name
Bump up YARDstick threshold