Replace NeoComplCache/Vim-GitGutter with YouCompleteMe/Vim-Signify

Remove:
 * NeoComplCache and SuperTab
 * Vim-GitGutter

Add:
 * YouCompleteMe
 * Vim-Signify
Add BufTabs and NERDTree Tabs

Add:
 * BufTabs and NERDTree Tabs
 * Add shortcuts to next/prev buffer ( <leader>; and <leader>' )