feat(Effect): Support decorating a class method

`@Effect()` decorator can now decorate a method in addition to a property

```ts
@Injectable() 
export class AuthEffects {
  constructor(private updates$) { }

  @effect() login() {
    return this.updates$.whenAction(...).mergeMap(...);
  }
}
```

Author: christophercr

lib/effects.ts

@@ -5,7 +5,14 @@ import { getEffectKeys } from './metadata';
 import { flatten } from './util';
 
 export function mergeEffects(...instances: any[]): Observable<any> {
-  const observables = flatten(instances).map(i => getEffectKeys(i).map(key => i[key]));
+  const observables: Observable<any>[] = flatten(instances).map((i: any): any => getEffectKeys(i).map(
+    (key: string): Observable<any> => {
+        if (typeof i[key] === 'function') {
+            return i[key]();
+        }
+        return i[key];
+    }
+  ));
 
   return Observable.merge(...flatten(observables));
 }

spec/effects.spec.ts

@@ -10,10 +10,11 @@ describe('mergeEffects', function() {
       @Effect() a = Observable.of('a');
       @Effect() b = Observable.of('b');
       @Effect() c = Observable.of('c');
+      @Effect() d() { return Observable.of('d'); }
     }
 
     const mock = new Fixture();
-    const expected = ['a', 'b', 'c'];
+    const expected = ['a', 'b', 'c', 'd'];
 
     mergeEffects(mock).toArray().subscribe({
       next(actual) {
@@ -37,9 +38,13 @@ describe('mergeEffects', function() {
       @Effect() c = Observable.of('c');
     }
 
-    const expected = ['a', 'b', 'c'];
+    class Fourth {
+      @Effect() d() { return Observable.of('d'); }
+    }
+
+    const expected = ['a', 'b', 'c', 'd'];
 
-    mergeEffects([ new First(), new Second(), new Third() ]).toArray().subscribe({
+    mergeEffects([ new First(), new Second(), new Third(), new Fourth() ]).toArray().subscribe({
       next(actual) {
         expect(actual).toEqual(expected);
       },