Using Python callbacks for logging within libmseed.

ms_log() will now properly raise warnings and errors. An issue is that
now everything with a level of >= 2 will be an exceptions and everything
below a warning. There is no lower info-like level. Thus it will either
be an exception or a warning. I think this is acceptable for almost all
use cases.
Adjusting logging levels.
Fixing the previous logging facility.

It is now able to distinguish between log messages, warnings, and
errors.
Adjusting log level.
Adjusting tests to new mseed logging.
Adding newline for the sake of consistency.
Stripping output originating in C
Merge pull request #679 from krischer/mseed_log

libmseed logging facility now uses Python callbacks