Zchunk update

It's been a number of weeks since my last update, so I thought I'd let everyone know where things are at. I've spent most of these last few weeks reworking zchunk's API to make it easier to use and more in line with what other compression tools use, and I'm mostly happy with it now. Writing a simple zchunk file can be done in a few lines of code, while reading one is also simple. I've also added zchunk support to createrepo_c (see https://github.com/jdieter/createrepo_c), but I haven't yet created a pull request because I'm not sure if my current implementation is the best method. My current effort only zchunks primary.xml, filelists.xml and other.xml and doesn't change the sort order. The one area of zchunk that still needs some API work is the download and chunk merge API, and I'm planning to clean that up as I add zchunk support to librepo. Some things I'd still like to add to zchunk: * A python API * GPG signatures in addition to (possibly replacing) overall data checksum * An expiry field? (I'm obviously thinking about signed repodata here) * Tests * More tests * Other arch testing (it's currently only tested on x86_64) I'd welcome any feedback or flames. Jonathan