sync: Store hashes of item content

[untitaker]

This should prevent vdirsyncer from unnecessarily entering conflict resolution mode on e.g. filesystems that have funky mtimes.

I wonder whether vdirsyncer should issue performance warnings if etags change but the item content didn't.

Related to #127, brought up by @hobarrera

[untitaker]

Latest master actually does this. This means that you can touch local files randomly and vdirsyncer will still not hit the network because the content didn't change. It will, however, burn a lot of CPU power if you do this for a lot of files.