Remove buffer nav bindings, <leader>ns

vim-unimpaired provides bindings that accomplished the same task, making
the previous bindings unnecessary.
Remove unite.vim and it's bindings

I find it to be unnecessarily complicated to configure and full of
features I don't require.
Add and configure ctrlp

- configure ctrlp to use 'ag' for searching
Added agignore

- agignore defines a list of directories and files for 'ag' to ignore
  while searching, used by ctrlp (with ag) to ignore unwanted search
  results.