Added "express --paas heroku" option which adds package.json entries for heroku deployment to work out-of-the-box

I keep having to copy and paste the correct engines settings for package.json before deploying a new app to heroku. This pull request will make that easier. Simple code, although I may have over-thought the generality of the --paas switch.

----------------- Prior to this pull request the first deploy experience is as follows:

-----> Node.js app detected
-----> Resolving engine versions

       WARNING: No version of Node.js specified in package.json, see:
       https://devcenter.heroku.com/articles/nodejs-versions

       WARNING: The default version of Node.js and npm on Heroku will begin
       tracking the latest stable release starting September 1, 2012.

       Using Node.js version: 0.4.7
       Using npm version: 1.0.106
-----> Fetching Node.js binaries
-----> Vendoring node into slug
-----> Installing dependencies with npm
       npm ERR! error installing express@3.0.6 Error: Unsupported
       npm ERR! error installing express@3.0.6     at checkEngine (/tmp/node-npm-5DgI/lib/install.js:493:14)
       npm ERR! error installing express@3.0.6     at Array.0 (/tmp/node-npm-5DgI/node_modules/slide/lib/bind-actor.js:15:8)
       npm ERR! error installing express@3.0.6     at LOOP (/tmp/node-npm-5DgI/node_modules/slide/lib/chain.js:15:13)
       npm ERR! error installing express@3.0.6     at chain (/tmp/node-npm-5DgI/node_modules/slide/lib/chain.js:20:4)
       npm ERR! error installing express@3.0.6     at installOne_ (/tmp/node-npm-5DgI/lib/install.js:470:3)
       npm ERR! error installing express@3.0.6     at installOne (/tmp/node-npm-5DgI/lib/install.js:411:3)
       npm ERR! error installing express@3.0.6     at /tmp/node-npm-5DgI/lib/install.js:347:9
       npm ERR! error installing express@3.0.6     at /tmp/node-npm-5DgI/node_modules/slide/lib/async-map.js:54:35
       npm ERR! error installing express@3.0.6     at Array.forEach (native)
       npm ERR! error installing express@3.0.6     at /tmp/node-npm-5DgI/node_modules/slide/lib/async-map.js:54:11
       npm ERR! error rolling back express@3.0.6 Error: ENOTEMPTY, Directory not empty '/tmp/build_2tgwfkdt0s1k3/node_modules/express'
       npm ERR! Unsupported
       npm ERR! Not compatible with your version of node/npm: connect@2.7.2
       npm ERR! Required: {"node":">= 0.5.0"}
       npm ERR! Actual:   {"npm":"1.0.106","node":"0.4.7"}