Provide Gumshoe functionality through Gumshoe instance

[lwoodson]

The current logic of accessing a Gumshoe instance

Gumshoe.get()...

does not work well with dependency injection. We got around this in data services with a custom Guice provider, but this should not be necessary. You should be able to create a Gumshoe instance, pass it around and have it work with thread local contexts and the like.

• The Gumshoe class now has thread-local management at the static/class level. It should be pushed down to the instance level.
• The context stack handling that happens at the instance level needs to be extracted to a new class that is managed by a Gumshoe instance.
• The existing developer interface into gumshoe should be preserved. Gumshoe.get() should return a singleton Gumshoe instance.
• Add docs for using Gumshoe with dependency injection