Use `logging` instead of `print`

[coldfix]

Use the logging module for all output rather than print.

Rationale:

• is more flexible and can be configured from within user code (for example if the user wants all pymad related log messages to go to a separate output file)
• can automatically add time/module and other information to output
• has multiple log levels (info, warning, error,.. ) indicating severity of log mesages
• bonus feature (probably least important): compatibility between python{2,3} without extra from __future__ import print_function statement

I suggest to write any new output using the logging module. At some point we might want to convert old code.

[coldfix]

I implemented this in the associated pull-request.

I hope the CERN folks get MAD-X revision working again, so the Travis tests become useful once more.