Add enable_testing() to CMakeLists.txt.
And add the following existing unit test to cmake test system:
  atomic_unittest
  date_unittest
  exception_test
  fileutil_test
  gzipfile_test
  logstream_test
  timestamp_unittest
  timezone_unittest
  buffer_unittest
  buffer_cpp11_unittest
  inetaddress_unittest
Merge pull request #79 from weizili/patch-enable_testing2

Add enable_testing() to CMakeLists.txt.