readme: npm install -> sudo npm install

Maybe it should be instructed to use `sudo npm install` instead? Without it I got this: 

>Adams-MacBook-Pro:~ adam$ npm install -g bower

>npm http GET https://registry.npmjs.org/bower
npm http 200 https://registry.npmjs.org/bower
npm http GET https://registry.npmjs.org/bower/-/bower-1.2.8.tgz
npm http 200 https://registry.npmjs.org/bower/-/bower-1.2.8.tgz
npm ERR! Error: EACCES, mkdir '/usr/local/lib/node_modules/bower'
npm ERR!  { [Error: EACCES, mkdir '/usr/local/lib/node_modules/bower']
npm ERR!   errno: 3,
npm ERR!   code: 'EACCES',
npm ERR!   path: '/usr/local/lib/node_modules/bower',
npm ERR!   fstream_type: 'Directory',
npm ERR!   fstream_path: '/usr/local/lib/node_modules/bower',
npm ERR!   fstream_class: 'DirWriter',
npm ERR!   fstream_stack: 
npm ERR!    [ '/usr/local/lib/node_modules/npm/node_modules/fstream/lib/dir-writer.js:36:23',
npm ERR!      '/usr/local/lib/node_modules/npm/node_modules/mkdirp/index.js:37:53',
npm ERR!      'Object.oncomplete (fs.js:107:15)' ] }
npm ERR! 
npm ERR! Please try running this command again as root/Administrator.

>npm ERR! System Darwin 12.5.0
npm ERR! command "node" "/usr/local/bin/npm" "install" "-g" "bower"
npm ERR! cwd /Users/adam
npm ERR! node -v v0.10.21
npm ERR! npm -v 1.3.11
npm ERR! path /usr/local/lib/node_modules/bower
npm ERR! fstream_path /usr/local/lib/node_modules/bower
npm ERR! fstream_type Directory
npm ERR! fstream_class DirWriter
npm ERR! code EACCES
npm ERR! errno 3
npm ERR! stack Error: EACCES, mkdir '/usr/local/lib/node_modules/bower'
npm ERR! fstream_stack /usr/local/lib/node_modules/npm/node_modules/fstream/lib/dir-writer.js:36:23
npm ERR! fstream_stack /usr/local/lib/node_modules/npm/node_modules/mkdirp/index.js:37:53
npm ERR! fstream_stack Object.oncomplete (fs.js:107:15)
npm ERR! 
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/adam/npm-debug.log
npm ERR! not ok code 0