support etags

add TAGS target to Makefile to support etags.

Signed-off-by: Yang Bai <hamo.by@gmail.com>
Comments on the tags target

Added comments to explain the target tags and TAGS,
also ignore the output file of ctags.

Signed-off-by: Yang Bai <hamo.by@gmail.com>
Merge pull request #40 from hamo/support_etags

support etags