Angular Nature adds the same Libs into .tern-project multiple times when...

... configured

At the moment the case can be only when manually removing the Angular Nature from .project file and then using "Convert to AngularJS Project". There is no check on that library is to be added already exists on Tern Project. As such, when converting a project into AngularJS Project, if .tern-project is already exist on that project, the "ecma5" and "browser" libraries are duplicated in .tern-project file and in memory (TernProject's JSONObject).

Also, I need the changes made in this fix in order to properly configure the .tern-project file in case of AngularJS-eclipse is adopted to a nature other than Angular Nature. (See: https://github.com/angelozerr/angularjs-eclipse/pull/55)

Signed-off-by: vrubezhny <vrubezhny@exadel.com>