Patch guice library to fix NPE in some corner cases

Currently used guice version (3.0) has a bug which triggers an NPE when
2 modules add different bindings to a same instance.
This is fixed by commit 6b7e7187bd07 in guice repository, scheduled to be
included in Guice 3.1 release.
See http://code.google.com/p/google-guice/issues/detail?id=473 for details.
Since Guice 3.1 library is bigger and includes quite significant changes,
out of compatibility concerns we just patch our current Guice version (3.0)
with this patch.
Merge pull request #8 from guillaume-mixi/master