Added Timestamp::Min/Max. Added poco_assert_msg and poco_assert_msg_dbg.

Added Timestamp::Min/Max which help define extreme timestamps, useful to initialize variables, especially for comparison.

Added NOMINMAX as a preprocessor define for Windows. Otherwise some Windows headers define min/max as macros which interferes with std::numeric_limits::min/max

Added poco_assert_msg and poco_assert_msg_dbg, which are equivalent to poco_assert and poco_assert_dbg, respectively, but support an extra text parameter.