Add `can_push` option to Deploy Keys (#146)

Fixes #145.

This adds the `can_push` option to the AddDeployKeyOption and DeployKey structs, in line with what is supported by GitLab.

deploy_keys.go

@@ -36,6 +36,7 @@ type DeployKey struct {
 	Title     string     `json:"title"`
 	Key       string     `json:"key"`
 	CreatedAt *time.Time `json:"created_at"`
+	CanPush   *bool      `json:"can_push"`
 }
 
 func (k DeployKey) String() string {
@@ -99,6 +100,7 @@ func (s *DeployKeysService) GetDeployKey(pid interface{}, deployKey int, options
 type AddDeployKeyOptions struct {
 	Title   *string `url:"title,omitempty" json:"title,omitempty"`
 	Key     *string `url:"key,omitempty" json:"key,omitempty"`
+	CanPush *bool   `url:"can_push,omitempty" json:"can_push,omitempty"`
 }
 
 // AddDeployKey creates a new deploy key for a project. If deploy key already