Cleaning vem

Related to tickets #72 and #73. I do not close the tickets because there is still work to do on these tickets.
It's a really bad idea to log in a file. Each time one runs unit tests, it creates a new log file in the folder where the `pyhrf_maketests` command is executed.
You should use the [logging](https://docs.python.org/2/library/logging.html) module from the standard library. In fact for now it is not implemented at all in pyhrf but we need to replace the homemade logging module (`_verbose`) by the former (good practice to start now!).