Make AngularJS-eclipse 'convert project ...' action bevare of that Tern....

...java

is able to treat a custom project nature as Tern nature.

If some project has a nature that is configured to be treated as Tern nature, then
that nature is used as a Tern nature when converting to AngularJS project instead of adding Tern nature.

depends on: https://github.com/angelozerr/tern.java/pull/38

Signed-off-by: vrubezhny <vrubezhny@exadel.com>