Refined http get/post interfaces to use Option.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/17076
Updated http tests in libprocess for the new get/post interfaces.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/17077
Refactored implicit conversions of Some, None, and Error.

Review: https://reviews.apache.org/r/17089
Added a conversion constructor to Future.

Review: https://reviews.apache.org/r/17090
Added overload of numify.

Review: https://reviews.apache.org/r/17091
Added some missing includes.

Review: https://reviews.apache.org/r/17092
Bug in http::request missing os::close.

Review: https://reviews.apache.org/r/17093