Add ability to redirect output into a file

[karras]

It would be nice to have a command line parameter to redirect the (debug) output of pyaptly / aptly into a separate file. Example:

$ pyaptly --debug --log output.log -c trusty.yml snapshot update

What do you think?

[ganwell]

We implemented the basics for that in #82. There is no actual cli-option for that yet. Note also, the tool is now silent unless something goes wrong.