initialize AuthButtonsCtr so that oauth2 library is eagerly loaded at startup

this avoids popup blocker. fix for https://github.com/Rise-Vision
/common-header/issues/302
Merge pull request #306 from settinghead/master

initialize AuthButtonsCtr so that oauth2 library is eagerly loaded at startup
ng-gapi-loader modularization
Merge pull request #307 from settinghead/chore/ng-core-api-client

ng-gapi-loader modularization
ng-gapi-loader modularization