Add Makefile taget 'etags' to create emacs compatible tags files

-- exuberant-ctags doesn't make emacs compatible tags files by default,
   so adding a new make target to make etags files on demand
Add tags files to .gitignore

-- ignore .tags and .etags using .gitignore.
Merge pull request #18 from 17Genr/master

Add etags make target and .gitignore tags files