Export package version

Exports the current package version from the root level. This allows
users of this module to read the version of the library that has been
loaded.

packages/opencensus-core/src/index.ts

@@ -53,3 +53,7 @@ export * from './stats/types';
 // logger
 import * as logger from './common/console-logger';
 export {logger};
+
+// version
+const version: string = require('../package.json').version;
+export { version };