RB Gateway currently only has a single namespace that is getting very

polluted. Because of this, methods that should be private aren't, since

they are accessible within the declared namespace (package main).
This is the first of a series of patches to split this monolithic

package into sub-packages. This patch tackles the repositories

(née repository.go and git_repository.go) package, as well as adds a

helpers package which contains all the test setup code that is used

across package boundaries.
Ideally, this would only have involved the testing infrastructure

changes, but because of how go imports work, we cannot import

github.com/reviewboard/rb-gateway from .../rb-gateway/helpers to

access Repository et al. because rb-gateway is a binary and not a

library.

Ran unit tests.