lib/download.c: add basic support for vcdiffs
lib/download.c: use tempfile while decoding new file from binary diff.

this prevents corrupted files when filename == basefile.
lib/download.c: debug output for binary diffs
bin/xbps-uhelper: fix flags for xfetch
lib/download.c: delete vcdiffs after applying; cleanups.
lib/repo_sync.c: use xbps_fetch_delta instead of xbps_fetch_file
NEWS: announce xdelta support
Merge pull request #52 from Gottox/xdelta

support for fetching file deltas for repo_sync.