Better debug support

When something doesn't work, the internal data structure of Guice is confusing and toString() needs additional tools (like an external text editor) because it's huge.

I would like to suggest a couple of changes to make debugging easier:
1. Support to assign a name or ID to an injector. That would help to distinguish them when I have more than one.
2. An easy way to dump the high-level structures:
1. The injector hierarchy
2. All the modules which were added per injector
3. There is a graphviz based tool to dump the bindings; add a similar tool which dumps to XML.