Support setting commit status

Support commit status updates.  Commit status can be set by a POST to
the appropriate commit URL.  The status can be updated by a subsequent
POST to the same URL with the same `name` and `ref`, but different
values for `state`, `description`, etc.

Note: Listing the commit statuses is not yet supported.  This is done
through a different path on the server, under the `repository` path.

Example of use from the CLI:

    # add a build status to a commit
    gitlab project-commit-status create --project-id 2 \
        --commit-id a43290c --state success --name ci/jenkins \
        --target-url http://server/build/123 \
        --description "Jenkins build succeeded"

1 files changed, 17 insertions, 0 deletions

diff --git a/gitlab/objects.py b/gitlab/objects.py
index 2ab2a52..baffec8 100644
--- a/gitlab/objects.py
+++ b/gitlab/objects.py
@@ -681,6 +681,22 @@ class ProjectCommitManager(BaseManager):
     obj_cls = ProjectCommit
 
 
+class ProjectCommitStatus(GitlabObject):
+    _url = '/projects/%(project_id)s/statuses/%(commit_id)s'
+    canUpdate = False
+    canDelete = False
+    requiredUrlAttrs = ['project_id', 'commit_id']
+    requiredCreateAttrs = ['state']
+    optionalCreateAttrs = ['description', 'name', 'ref', 'target_url']
+    requiredGetAttrs = []
+    requiredUpdateAttrs = []
+    requiredDeleteAttrs = []
+
+
+class ProjectCommitStatusManager(BaseManager):
+    obj_cls = ProjectCommitStatus
+
+
 class ProjectKey(GitlabObject):
     _url = '/projects/%(project_id)s/keys'
     canUpdate = False
@@ -961,6 +977,7 @@ class Project(GitlabObject):
     managers = [
         ('branches', ProjectBranchManager, [('project_id', 'id')]),
         ('commits', ProjectCommitManager, [('project_id', 'id')]),
+        ('commitstatuses', ProjectCommitStatusManager, [('project_id', 'id')]),
         ('events', ProjectEventManager, [('project_id', 'id')]),
         ('files', ProjectFileManager, [('project_id', 'id')]),
         ('forks', ProjectForkManager, [('project_id', 'id')]),