update vimrc for vim74 compatibility

all about line number
1. add norelativenumber! in key map <F2>:
nnoremap <F2> :set norelativenumber! nonumber! number?<CR>
2. when change status between number and relativenumber, shutdown the other one first:
au FocusLost * :set norelativenumber number
au FocusGained * :set nonumber relativenumber
" 插入模式下用绝对行号, 普通模式下用相对
autocmd InsertEnter * :set norelativenumber number
autocmd InsertLeave * :set nonumber relativenumber
function! NumberToggle()
  if(&relativenumber == 1)
    set norelativenumber number
  else
    set nonumber relativenumber
  endif
endfunc
nnoremap <C-n> :call NumberToggle()<cr>
update relativenumber-related setting: 1. set number always on; 2. repair F2 map, it deals with all kinds of number and relativenumber status now
Merge pull request #14 from Garnel/master

update vimrc for vim74 compatibility