Refactors. Fixes #4.

* break out json and test submodules
* move some tests out of `json_helpers_test.go` and into a new `session_test.go` because they are relevant to geotriggers sessions specifically
* add a new `environment` struct as a field on `application` and `device` for easier switching to test env when testing those files.
* new `client_test.go` for testing the app and device constructors from `client.go`, as those are no longer tested down in the app and device test files