[Sprint : 40] XD-2427 Use repo.spring.io as NPM repository

* Add `.npmrc`
* Add information to `README.md`
When building `gradlew cleanUI setupUI`, I am running into this:

```
npm WARN package.json xd@1.0.0 No repository field.
npm WARN deprecated grunt-ngmin@0.0.3: use grunt-ng-annotate instead
npm ERR! Error: Bad Request
npm ERR!     at errorResponse (/Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/lib/node_modules/npm/lib/cache/add-named.js:260:10)
npm ERR!     at /Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/lib/node_modules/npm/lib/cache/add-named.js:120:12
npm ERR!     at saved (/Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/lib/node_modules/npm/node_modules/npm-registry-client/lib/get.js:167:7)
npm ERR!     at Object.oncomplete (fs.js:107:15)
npm ERR! If you need help, you may report this *entire* log,
npm ERR! including the npm and node versions, at:
npm ERR!     <http://github.com/npm/npm/issues>

npm ERR! System Darwin 13.2.0
npm ERR! command "/Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/bin/node" "/Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/lib/node_modules/npm/bin/npm-cli.js" "install"
npm ERR! cwd /Users/igopinatha/workspace/git/spring-projects/spring-xd/spring-xd-ui
npm ERR! node -v v0.10.33
npm ERR! npm -v 1.4.28
npm ERR! code E400
npm ERR!
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/igopinatha/workspace/git/spring-projects/spring-xd/spring-xd-ui/npm-debug.log
npm ERR! not ok code 0
:spring-xd-ui:npmInstall FAILED

FAILURE: Build failed with an exception.
```

the npm debug log says:
```
6 verbose install     'jshint-stylish',
:$
1011 silly registry.get cb [ 304,
1011 silly registry.get   { date: 'Mon, 15 Dec 2014 23:25:52 GMT',
1011 silly registry.get     server: 'nginx',
1011 silly registry.get     'x-artifactory-id': 'spring1-use-1d-springsource',
1011 silly registry.get     'x-node': 'nginx2-use-1d',
1011 silly registry.get     connection: 'keep-alive' } ]
1012 verbose etag http://repo.spring.io/api/npm/npmjs/grunt-contrib-nodeunit from cache
1013 silly lockFile 3261ce55-grunt-contrib-nodeunit-0-4-1 grunt-contrib-nodeunit@0.4.1
1014 silly lockFile 3261ce55-grunt-contrib-nodeunit-0-4-1 grunt-contrib-nodeunit@0.4.1
1015 http 400 http://repo.spring.io/api/npm/npmjs/grunt-protractor-runner
1016 silly registry.get cb [ 400,
1016 silly registry.get   { 'content-type': 'application/json',
1016 silly registry.get     date: 'Mon, 15 Dec 2014 23:25:52 GMT',
1016 silly registry.get     server: 'nginx',
1016 silly registry.get     'x-artifactory-id': 'spring1-use-1d-springsource',
1016 silly registry.get     'content-length': '112',
1016 silly registry.get     connection: 'keep-alive' } ]
1017 silly lockFile dba6b758-grunt-protractor-runner-1-1-4 grunt-protractor-runner@1.1.4
1018 silly lockFile dba6b758-grunt-protractor-runner-1-1-4 grunt-protractor-runner@1.1.4
1019 error Error: Bad Request
1019 error     at errorResponse (/Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/lib/node_modules/npm/lib/cache/add-named.js:260:10)
1019 error     at /Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/lib/node_modules/npm/lib/cache/add-named.js:120:12
1019 error     at saved (/Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/lib/node_modules/npm/node_modules/npm-registry-client/lib/get.js:167:7)
1019 error     at Object.oncomplete (fs.js:107:15)
1020 error If you need help, you may report this *entire* log,
1020 error including the npm and node versions, at:
1020 error     <http://github.com/npm/npm/issues>
1021 error System Darwin 13.2.0
1022 error command "/Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/bin/node" "/Users/igopinatha/.gradle/nodejs/node-v0.10.33-darwin-x64/lib/node_modules/npm/bin/npm-cli.js" "install"
1023 error cwd /Users/igopinatha/workspace/git/spring-projects/spring-xd/spring-xd-ui
1024 error node -v v0.10.33
1025 error npm -v 1.4.28
1026 error code E400
1027 verbose exit [ 1, true ]
```