TL logging API harmonization - Consistent with TF logging API

[DEKHTIARJonathan]

This PR aims to make logging:

• consistent with TF logging API
• thread-safe

It has been highly inspired by https://www.tensorflow.org/api_docs/python/tf/logging

Now it is possible to do the followings:

import tensorlayer as tl

tl.logging.set_verbosity(tl.logging.DEBUG)
tl.logging.set_verbosity(tl.logging.INFO)
tl.logging.set_verbosity(tl.logging.WARN)
tl.logging.set_verbosity(tl.logging.ERROR)
tl.logging.set_verbosity(tl.logging.FATAL)

tl.logging.debug("This is a debug message")
tl.logging.error("This is an error message")
tl.logging.fatal("This is a fatal error message")
tl.logging.info("This is an information message")
tl.logging.warn("This is a warning message")