Restructured Activity Service and Event files.
Restructured Git Data Service files.
Restructured Issues Service files.
Restructured Orgs Service Files.
Restructured Pull Request Service files.
Restructured Repository Service files.
Restructured Users Service files.
Merged webhooks.go into repos_hooks.go
Moved timestamp implementation into its own file.

This was done so that timestamp_test.go would have a corresponding file that
could easily be found. It did not make sense to move timestamp_test.go into
github_test.go if we are trying to move to a more specific source structure.
Various gofmt fixes from restructuring.
Updated CONTRIBUTING.md to describe new naming scheme.
minor cleanup from file refactor

mostly just typos and whitespace