Make Tern.java to be able to treat a custom project nature as Tern nature

This will allow Tern.java to be seamlessly integrated to other products.

Other products may declare a custom project nature to be treated as a Tern nature by adding the following extension into some plugin.xml:

	<extension 
		point="tern.eclipse.ide.core.ternNatureAdapters"
		id="com.example.product.tern.ternNatureAdapters"
		name="come.example.product.tern.ternNatureAdapters">
		
		<ternAdaptToNature 
			id = "com.example.product.someNature"
			name = "Some example project nature ID" />
	</extension>

Signed-off-by: vrubezhny <vrubezhny@exadel.com>