Added NestedContext, and made changes to binding and injection classes t...

Hi!  I don't mean this as a real pull request, but merely as a way of showing you something I did with strange.  Basically, I had to port a project from AS3 to C# that used RobotLegs.  It was incredibly helpful that strange was so similar to RL, and I generally thought strange was easier to use.  However, RL does one thing that strange did not: allow for nested contexts.  The cross-context thing just doesn't work the same.  So, we added our own NestedContext child of MVCSContext.  The heart of it is in Binder, where, if a binding cannot be found it checks to see if it has a parent Binder and calls getBinding on the parent.  It seems to replicate RL well enough (I don't know of any problems).  Maybe it's rare for RL users to use that system, but this project used it extensively (think thousands of contexts!)

Also, thanks for making strange, it really saved us a lot of time!