add support for recursive abstract class binding via BindAllBase()

This pull request addresses the additional functionality discussed in [this StackOverflow question](http://stackoverflow.com/questions/18091906/bindbase-only-binds-immediate-ancestors) and [issue 11](https://github.com/ninject/ninject.extensions.conventions/issues/11). It adds a binding type `BindAllBase()` that recursively binds any abstract classes in the inheritance tree, unlike `BindBase()` which only binds the immediate ancestor.