Refactor: replace "render :json = graph.to_json" to view template(show.json.erb).

Because model shouldn't know about view logic.
Refactor: grouping parent and their space by including array.
Refactor: rename module and class names.

* Module: Graph -> Network
* Class: JsonBuilder -> Graph
Refactor: clean up models.

* Network::Commit
** Removing unnecessary accessors.
** Removing add_refs methods.
* Network::Graph
** Removing unnecessary accessors.
** The 3 times loop of commits don't need.
Refacor: removing the times array, because that is same with @commits array.
Refactor: change the map hash from a local variable to private variable.
Refactor: Removing the duplicated code.
Refactor: Removing a if statement, because the nil value is already removed in the view template.
API: system hooks API functions and documentation updated

* updated system hooks documentation and code comments
* fixed access to system hooks if no user given resulting in a `500 Server Error`
* added tests
Merge branch 'master' into api/system_hooks_adjustments
better logo images for header
Replace current Wiki system with Gollum Wikis.

This commit replaces the old database backed Wiki system with the
excellent Gollum git based Wiki system.

The UI has been updated to allow for utilizing the extra features
that Gollum provides. Specifically:

* Edit page now allows you to choose the content format.
* Edit page allows you to provide a commit message for the change.
* History page now shows Format, Commit Message, and Commit Hash.
* A new Git Access page has been added with the Wiki Repo URL.
* The default page has been changed to Home from Index to match
the Gollum standard.

The old Wiki model has been left in tact to provide for the
development of a migration script that will move all content stored
in the old Wiki system into new Gollum Wikis.
Merge pull request #3169 from hiroponz/refactor-network-graph

Refactor network graph
Move groups and teams to tabs on dashboard. Remember tab with cookie
Group and team rss is valid now
add coveralls.io to travis builds
fixed travis env, added coverage badge
Add Spinach coverage for Gollum Wiki system and correct the Delete link.

The previously failing Spinach steps have been fixed with this commit.
I have also added new steps that cover the entire usage of the Wiki
system.

The new Spinach steps revealed a minor bug in the Delete page process.
The path for the "Delete this page" button was previously set to
`project_wikis_page(@project, @wiki)` when it should have been
using the singular `project_wiki_page(@project, @wiki)` path helper.

The link has been corrected and all steps are now passing.
Create Wiki migration task.

This commit adds a new Rake task for migrating all of your existing
Wiki content from your database into new Gollum repositories.

The bulk of the logic happens within the `WikiToGollumMigrator`
class which is decently test covered and located in the lib directory.

The new Rake task can be executed by running:

   `bundle exec rake gitlab:wiki:migrate`

It will output a nice log of every project that it migrates along
with success or failure messages.

I have used it on my own installation to migrate my Wikis successfully.
Merge pull request #3170 from Asquera/api/system_hooks_adjustments

API: system hook request functions and documentation updated