Add new method Context.ref() for obtaining a ContextReference.
Updated Vaadin versions.
Move the Maven archive descriptor to the default location.
Add support for Vaadin 7, remove support for Vaadin 6.
Revert "Add new method Context.ref() for obtaining a ContextReference."

This reverts commit 10569ac5405c69e91463feee6b3877f63221fda8.

References can already be created by Context.from(Component).ref(Type).

The method Context.ref() was orginally added to prevent an unnecessary
instantiation of ContextLocator for each invocation. The additional
instantiation can still be avoided by using ContextReference directly.
Bump version to 0.2.