refactor(core): separate reflective injector from Injector interface

BREAKING CHANGE:
- Injector was renamed into `ReflectiveInjector`,
  as `Injector` is only an abstract class with one method on it
- `Injector.getOptional()` was changed into `Injector.get(token, notFoundValue)`
  to make implementing injectors simpler
- `ViewContainerRef.createComponent` now takes an `Injector`
  instead of `ResolvedProviders`. If a reflective injector
  should be used, create one before calling this method.
  (e.g. via `ReflectiveInjector.resolveAndCreate(…)`.

modules/benchpress/test/sampler_spec.ts

@@ -22,7 +22,7 @@ import {
   Reporter,
   bind,
   provide,
-  Injector,
+  ReflectiveInjector,
   Options,
   MeasureValues
 } from 'benchpress/common';
@@ -65,7 +65,7 @@ export function main() {
         bindings.push(bind(Options.PREPARE).toValue(prepare));
       }
 
-      sampler = Injector.resolveAndCreate(bindings).get(Sampler);
+      sampler = ReflectiveInjector.resolveAndCreate(bindings).get(Sampler);
     }
 
     it('should call the prepare and execute callbacks using WebDriverAdapter.waitFor',