Add client method to generate build

Adds a method to the client to generate a build, given a buildConfig and modifies the webhook controller to use the new client method.
Yeah, error handling should be the consumer of this code's responsibility.

----- Original Message -----
> > @@ -140,6 +141,22 @@ func (c *Client) CreateBuild(ctx kapi.Context, build
> > *buildapi.Build) (result *b
> >  	return
> >  }
> >  
> > +// GenerateBuild creates a new build based on a given BuildConfig.
> > Optionally a SourceRevision for the new
> > +// build can be specified
> > +func (c *Client) GenerateBuild(ctx kapi.Context, bc *buildapi.BuildConfig,
> > r *buildapi.SourceRevision) (result *buildapi.Build, err error) {
> 
> @soltysh - From the command line, sure... we can just specify the build
> config ID and we'll kick off the build for you. However, internally it made
> more sense to pass the buildconfig object since consumers like webhooks
> already have a buildconfig that they're looking at.
> 
> ---
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> https://github.com/openshift/origin/pull/410/files#r20574431