XD-1229 + XD-1230 Create build infrastructure for AngularJS based UI

The Grunt-based build infrastructure is based on Yeoman's AngularJS support and then further customized. As part of `XD-1230`, this build process is then further integrated into the larger Spring XD Gradle build. Please see `README.md` for further details.

* Add AngularUI Router dependency
* Add UI-Router routes
* Add LESS support + UI Admin Styles
* Instead of customizing Bootstrap's `variables.less`, we instead import its styles into `main.less` and customize only the variables that changed. That way we can easily upgrade Bootstrap in the future
* Fix a `grunt-usemin` issue with images in CSS files
  - Use fix as detailed here: https://github.com/yeoman/yeoman/issues/824#issuecomment-29137057
  - See also issue: https://github.com/yeoman/grunt-usemin/issues/242
* Exclude vendor CSS, we already import the Bootstrap CSS via `main.less`
* Don't use port 8080 for Karma tests in order to minimize conflicts with Tomcat
* Add "Loading Indicator" support using `angular-busy`
* Add Job launch support + Growl support + define XD Server URL globally
* Use Bootstrap fonts from the distribution
* Improve Job launch support
* Update `.gitignore`
* Update Nodejs dependencies, fix test dependencies
  - Running `npm install` should install all relevant dependencies from `package.json`
* Add Gradle build support
* Import CSS files as LESS resources in `main.less`
  - Otherwise you run into minification issues as Grunt's nsmin does not consider those CSS files
* Remove old `admin-ui`
* Improve resolving of the admin-ui URLs
  - http://localhost:9393/admin-ui --> http://localhost:9393/admin-ui/#/jobs/definitions
  - http://localhost:9393/admin-ui/ --> http://localhost:9393/admin-ui/#/jobs/definitions
* Add task `cleanDist`
* Improve Gradle build - Depend on `spring-xd-dirt:backgroundAdminServer`
* Fix `xd.ui.home` in `application.yml`
  - `xd.ui.home` in `application.yml` should contain a trailing slash
* Add `grunt-bower-task` to Grunt build
  - Allow Bower to be called via Grunt
* Test that the UI builds with `gradle npmInstall installGrunt build`
* Update Node dependency versions
* Fix "Required config property "bower-install.app.src" missing"
  - Bower Install - In `Gruntfile.js`, change config `html: '<%= yeoman.app %>/index.html',` to `src: '<%= yeoman.app %>/index.html',`
* Add Proxying to XD Admin Server via Grunt build
  - Add https://github.com/drewzboto/grunt-connect-proxy
* Better error messages when pulling Job Definitions
* Add `.bower_cache` directory local to project dir
* Add Karma shell scripts
  - I find these shell scripts quite useful when writing unit and e2e tests as you can configure Karma to keep running as a daemon (automatically re-running the tests of code-changes do occur)
* Add Grunt task `testfiles` - useful to prepare Spring XD test files
* Add Karma Safari launcher
* Add Unit and E2E tests
* Remove unnecessary files
  - .gitattributes
  - app/.buildignore
  - app/robots.txt
* Remove old loading Gif `loading.gif`
  - Remove unused related CSS
* Ensure we use only single quotes in JS files
* Improve Favicon support including mobile
* Remove main.css + main.css.map (autogenerated)
* Make sure LESS creates CSS files when executing `grunt serve`
* Update `.gitignore`
* Change Grunt Server Port to 8000
  - Port 9000 conflicts with Sonar
* Improve Gradle Build - `gradle build` will:
  - Install (if needed) node
  - Install (if needed) grunt
  - Download node dependencies for XD UI
  - Run the Grunt build
* `gradle clean` will remove all unnecessary spring-xd-ui folders (e.g. local node modules, bower files etc)
* Run Spring XD UI Grunt build on Eclipse STS import
  - Add task `afterEclipseImport`
  - See for details: https://issuetracker.springsource.com/browse/STS-2059
* Add `README.md`
* Fix live-reloading of Html files
* Fix copying of images in `Gruntfile.js`

XD-1229 Code Review changes

* Remove `.htaccess`
* Run E2E as part of the Gradle test task
* For distribution, copy Admin UI to `spring-xd-ui/dist`
Polishing the UI tasks

- Make spring-xd-ui project independent and not run the build
  as part of 'gradle build'
- Added tasks 'setupUI' and 'cleanUI' that takes care of building/cleaning
  the UI project
- Removed an invalid URL from index.html