gzipped event files

[derNarr]

Make all event files gzipped with the internal gzip library and compression level 5 on default (around the best speed vs. size tradeoff).

Add to the documentation that these files can be viewed with zless and zcat...

[dekuenstle]

Duplicate of #17? @derNarr

[derNarr]

Partially, here it is more to save space on the hard drive. This is easy to do and might help according to speed as well. I would keep this one separate as it is easy to implement and clearly defined (in contrast to #17).