update fs.{read,write}File{,Sync} decls

In current (0.10.31) node, these functions take an options object as
their second parameter, not a string.  I'm not sure when this was
changed, although I think it happened many versions ago.

http://nodejs.org/api/fs.html#fs_fs_readfile_filename_options_callback
http://nodejs.org/api/fs.html#fs_fs_readfilesync_filename_options
http://nodejs.org/api/fs.html#fs_fs_writefile_filename_data_options_callback
http://nodejs.org/api/fs.html#fs_fs_writefilesync_filename_data_options

The string form has been left as an alternative for users who are using
old versions of node.