Chrono cleanup

[K-ballo]

This PR tackles the conversion from posix_time based interfaces to chrono based interfaces. Existing functionality is retained (but posix_time support should be deprecated). See #1190.

[sithhell]

Looks good so far!
How would support for std::chrono instead of boost::chrono look like?

[K-ballo]

Support for std::chrono instead of boost::chrono would consist in replacing boost::chrono with std::chrono, plus a few includes.

Supporting both at the same would be even easier, requiring just a macro to detect if it's available, and an extra constructor for steady_time_point and steady_duration.

[zao]

I mentioned it on IRC but it's probably good to persist here too.
Ubuntu 13.10 (or 13.04, memory is vague) had a libstdc++ in which some durations were 1000x as long as they should be. This means that you cannot universally replace Boost's Chrono with the standard one, even if the library claims to have one.

[hkaiser]

Looks good to me, however having support for both, std::chrono and boost::chrono would be nice.

[K-ballo]

Added support for std::chrono.