add singleton as source helper

[piercus]

Hello,

Following remarks in #35, #27 and #35 i was not finding a good way to inject singletons manually.

I'd like to have this design

IoC.use(IoC.singleton('settings', {foo : 'bar'}));

Hope it will help someone

[coveralls]

Coverage decreased (-1.6%) to 91.93% when pulling 799b061 on piercus:sources-singleton into d6af75b on jaredhanson:master.