Allow JSON response body to use symbols instead of strings

Ruby stdlib JSON lib provides the configuration option `symbolize_names`: http://ruby-doc.org/stdlib-2.3.0/libdoc/json/rdoc/JSON.html#method-i-parse

It would be really nice if `Unirest` provided a configuration option (either default or per-request) to hook into this, removing the need to manually symbolize the keys on the response body.