Summary: Added a target to Makefile called 'tags' that runs ctags and cscope on all *.cc and *.h file

Test Plan:
Run 'make tags'. Then start vim and do
:set tags=./tags
:cs add cscope.out

These commands should give you no error messages. You should then be able to access cscope db and ctags as normal in vim.

Reviewers: dhruba

Differential Revision: https://reviews.facebook.net/D11103