Missing Apache license header in some source files

_From [mcculls](https://code.google.com/u/108081409801028235828/) on April 04, 2012 18:32:04_

FYI, the Apache license header is missing from the following source files:

core/src/com/google/inject/internal/DelayedInitialize.java
core/src/com/google/inject/internal/Nullability.java
core/src/com/google/inject/spi/Toolable.java

extensions/throwingproviders/src/com/google/inject/throwingproviders/CheckedProvideUtils.java
extensions/throwingproviders/src/com/google/inject/throwingproviders/CheckedProviderWithDependencies.java

The header should be added to make it clear these files are licensed under the ASL.

_Original issue: http://code.google.com/p/google-guice/issues/detail?id=697_