* Unify logging facilities.
* Enhance `ConsoleLogger` to handle different verbosity.
* Override macros from `dmlc`.
* Don't use specialized gamma when building with GPU.
* Remove verbosity cache in monitor.
* Test monitor.
* Deprecate `silent`.
* Fix doc and messages.
* Fix python test.
* Fix silent tests.
On Unix systems, it's common for programs to read their input from stdin, and
write their output to stdout. Messages should be written to stderr, where they
won't corrupt a program's output, and where they can be seen by the user even
if the output is being redirected.
This is mostly a problem when XGBoost is being used from Python or from another
program.
