Fix byte compile warnings

 - load etags.el for 'tags-completion-table'
 - add group for this module
 - add group information for faces
 - use with-current-buffer instead of set-buffer + save-excursion
 - use let for free variables
 - fix typo erro -> error
Merge pull request #1 from syohex/fix-byte-compile

Fix byte compile warnings

 - load etags.el for 'tags-completion-table'
 - add group for this module
 - add group information for faces
 - use with-current-buffer instead of set-buffer + save-excursion
 - use let for free variables
 - fix typo erro -> error