Use `logging` instead of `print`

Use the [`logging`](http://docs.python.org/3.3/library/logging.html) 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.