use tabs in vim's neocomplcache plugin

neocomplcache-keymaps.vim

" Recommended key-mappings.
" <CR>: close popup and save indent.
inoremap <expr><CR>  neocomplcache#smart_close_popup() . "\<CR>"
" <TAB>: completion.
inoremap <expr><TAB>  pumvisible() ? "\<C-n>" : "\<TAB>"
" <C-h>, <BS>: close popup and delete backword char.
inoremap <expr><C-h> neocomplcache#smart_close_popup()."\<C-h>"
inoremap <expr><BS> neocomplcache#smart_close_popup()."\<C-h>"
inoremap <expr><C-y>  neocomplcache#close_popup()
inoremap <expr><C-e>  neocomplcache#cancel_popup()