Allow to specifiy a method to retrieve context.

Add the possibility to retrieve the context
by a method.
Merge pull request #1 from tawan/context_as_proc

Pass contexts as Proc objects.
Make overwritting of filtering access to action optional.

In some cases it is convenient to combine access filter to
a certain action of a controller. This was not possible
since an access filter for the same action would overwrite
any access filter to this action which had been defined earlier.

Make this behavior optional but default to overwitting.