Various bug fixes and enhancements

- Rename ggtags-cache-timestamp to ggtags-get-timestamp
- Remove use of user-error in ggtags-mode definition
- Highlight tag at point using idle timer for efficiency
- Command ggtags-find-tag learns to do what I mean
- Bind M-, to ggtags-find-tag-resume
- Teach ggtags-kill-file-buffers to look in GTAGSLIBPATH
- Be consistent in popping up windows
- Update README
* ggtags.el: improve handling of GTAGSLIBPATH in ggtags-tag-names
* ack: release v0.9

- remove emacs 23 support
- add a new command ack-mode-display-match
Remove ampc.

* ampc.el, ampc_tagger.cpp: Remove files.