Strip out std::chrono.

* [x] __TEST BEFORE MERGING!__
* [x] Bonus ball: test with clang 3.5
* [x] Thunderball: build (but not test) on [travis][] with clang 3.4 and gcc 4.6

__NOTE:__ The scope of this pull request has drifted slightly, as it also includes changes to allow `playd` to be built (but not tested, yet) by [travis][] on clang3.4 and gcc4.6.  The latter is lacking in its support of some C++11 features (notably `using`-style typedefs and `override`), so these have been removed.  The original message follows.  __(End of note.)__

All uses of `std::chrono` have been replaced with flat `uint64_t`s representing microseconds.  The [`UNITS`][] map now holds multipliers for converting from the units to microseconds.

This removes quite a bit of boilerplate and templates.  Needs to be tested before merge; this might break things.

[travis]: https://travis-ci.org/UniversityRadioYork/ury-playd
[`UNITS`]: https://github.com/UniversityRadioYork/ury-playd/blob/e6fe73a17ac5f8b07367172735700e49813c9d31/src/main.cpp#L79-L92
`override` was removed because gcc4.6 doesn't like it.