A Google timezone library.

This version is based on commit 2e86a776379ec6f2a60f4a6595a3b68cd3961cb4 from https://github.com/google/cctz.git

The following changes were made compared to the vanilla Google code:
  * the Time Zone Database may optionally be compiled into the library (just add a PEERDIR() to contrib/libs/cctz/tzdata subdirectory in your code);
  * some minor cleanups to avoid clang warnings;
  * the tests were modified to work with library/cpp/testing/unittest.