Upgrade guide

api changes from version below 1.0 to 1.x

• exists are changed to node-style arguments with callback. In previous version it uses memory as default repo to store keys, you can get result by return value but you can't in new version. seenreq uses process.nextTick to produce asynchronous callback. So be careful to change your code to get result in callback even if you use defualt memory repo.
• normalize return value is an object now, it looks like: {sign:"GET http://www.google.com\r\n",options:{key1:"val"}}, the sign is same as the returned string by normalize before.
• options.update is changed to options.rupdate to avoid duplicate, so you can place rupdate in request, e.g. {uri:"http://www.google.com", rupdate:false}. It also takes effect to place it in options.
• Use initialize before use exists