Support for std::numeric_traits

[Daryle Walker]

Although boost::rational uses std::numeric_limits in its implementation, it does not provide its own (partial) specialization of the standard class template. This means that generic numeric code cannot use rational in contexts that use the assistance of numeric_limits.

This has been brought to my attention by Michael Olea in his message <​http://lists.boost.org/boost-users/2013/08/79751.php> to the Users' list.

[awulkiew]

Since during the testing of some functions in Boost.Geometry I hit this problem I decided to help with it. I've attached a patch, not sure if all values are correct though.