[Boost-maint] [date_time] Restore to_time_t() function

Subject: [Boost-maint] [date_time] Restore to_time_t() function
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-05-31 12:53:45

This pull request:

https://github.com/boostorg/date_time/pull/13

restores to_time_t() function and its tests removed by commit

https://github.com/boostorg/date_time/commit/6636f49f13154555b3b9c9618fca5f03cf47a5b3