Updated vimrc

-Added cscope support (command abbreviations, etc)
-Changed leader key from default backslash to space
-Added a couple of new plugins, unite.vim, vimproc.vim,
 vim-togglelist, and vim-cpp-enhanced-highlight
-Added leader key t which replaces Command-T
-Lowered update time to 500 ms from 1000 ms
-Auto close preview window from ycm after you get out of insert