[WIP] Add option to cancel a build

this is a rough draft, I'm actively working on it.. so it may throw errors and it may have dirty code.

   - Add cancelling and cancelled status for builds
   - Remove pod and update builds to cancelled when a cancel event was triggered for the build
Cancelled is the most appropriate name


> On Nov 25, 2014, at 4:52 AM, Michal Fojtik <notifications@github.com> wrote:
> 
> In pkg/build/api/types.go:
> 
> > @@ -18,6 +18,9 @@ type Build struct {
> >  
> >  	// PodName is the name of the pod that is used to execute the build
> >  	PodName string `json:"podName,omitempty" yaml:"podName,omitempty"`
> > +
> > +	// CancelFlag describes if cancelling event was triggered for the build.
> > +	CancelFlag bool `json:"cancelFlag,omitempty" yaml:"cancelFlag,omitempty"`
> I would prefer Canceled or Canceling instead of CancelFlag
> 
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