Add LR for Tensorboard in LearningRateScheduler

[ChrisGll]

Summary

Add learning rate in LearningRateScheduler's log.

This is already done for the callback ReduceLROnPlateau, which is similar. The enables Tensorboard to display the learning rate. Changes done are identical to the implementation of ReduceLROnPlateau.

Related Issues

PR Overview

• This PR requires new unit tests [y/n] (make sure tests are included)
• This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• This PR is backwards compatible [y/n]
• This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[fchollet]

That's a good idea, thanks for the PR.