Support yaml variables

This MR introduces ability to define variables from YAML.

```yaml
variables:
  DB_NAME: postgres

test:
  script: echo $DB_NAME
```

The variables are passed using the same API as Secure Variables. The API introduces additional parameter: public. All variables defined in YAML are marked as public. The GitLab Runner when detects public variables will pass them to the services. This makes it easy to fine tune linked services to for example define database name.

```yaml
services:
- postgres

variables:
  POSTGRES_DB: gitlab
```

The above example will run [postgres](https://registry.hub.docker.com/u/library/postgres/) and pass POSTGRES_DB to postgres container making it to create `gitlab` database instead of default `postges`.

**Note:** All variables will passed to all service containers. It's not designed to distinguish which variable should go where.

/cc @sytses @vsizov @dzaporozhets 

See merge request !227

Fix service testing

Fixes:
- https://gitlab.com/gitlab-org/gitlab-ci/issues/248
- https://gitlab.com/gitlab-org/gitlab-ci/issues/208

/cc @vsizov 

See merge request !221

Fix project API listing returning empty list when first projects are not added to CI

The CI prefers to receive the projects added to CI first

/cc @vsizov 

See merge request !225

The CI prefers to receive the projects added to CI first

Commits with [ci skip] have special skipped status

![Screen_Shot_2015-07-30_at_14.02.50](https://gitlab.com/gitlab-org/gitlab-ci/uploads/f200af385679b564cd15d730bd4ccd20/Screen_Shot_2015-07-30_at_14.02.50.png)

+ Commits with [ci skip] are marked as skipped
+ Commits without builds are marked as skipped

Resolution for:
+ https://dev.gitlab.org/gitlab/gitlab-ci/issues/264
+ https://dev.gitlab.org/gitlab/gitlab-ci/issues/266

This requires CE update:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1073

/cc @vsizov @sytses 

See merge request !216

Rename type to stage

However, make the `type` and `types` as alias for `stage` and `stages`.

/cc @vsizov @sytses 

See merge request !222

Remove unused entities



See merge request !224

Refactor access_token usage for API requests

This MR refactors GitLab API usage to use either access_token or private_token. It also allows to use access_token when executing GitLab CI API.

/cc @vsizov @dzaporozhets 

See merge request !226

Fix truncate_first_line if message is nil

Fixes https://gitlab.com/gitlab-org/gitlab-ci/issues/255

/cc @jacobvosmaer 

See merge request !217

Use avatars from GitLab

Rewrite Gravatar URLs if needed in order to have images in different size

/cc @dzaporozhets @vsizov 

See merge request !220

Rewrite Gravatar URLs if needed in order to have images in different size

Make YAML validation stricter

/cc @vsizov 

See merge request !218

Randomize test database

/cc @vsizov 

See merge request !219