Pinning the angular2 dependency, and including licenses. (#893)

accessible/libs/README

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+This folder contains the following dependencies for accessible Blockly:
+
+* Angular2 (angular2-all.umd.min.js, angular2-polyfills.min.js)
+* RxJava (Rx.umd.min)
+
+Used for data binding between the core Blockly workspace and accessible Blockly.
+RxJava is required by Angular2.
+Fetched from https://code.angularjs.org/
+The current version is 2.0.0-beta.16.
+
+* ES6 Shim
+
+Required by Angular2, for Javascript files.
+Fetched from https://github.com/paulmillr/es6-shim
+The current version is 0.35.1.