Fix #77: Robustify ggtags-update-tags-single for remote files
New variable ggtags-use-sqlite3 to enable sqlite3 storage
Include NEWS for current release in header comments
Fix #79: Make ggtags-update-tags run in the background

Invoking `ggtags-update-tags' interactively now runs `global -u' in
the background for large projects whose GTAGS size is +10M per
ggtags-oversize-limit.