Improve flow typing for action method

This commit adds better flow typing for the `action()` method. Using
flowtype's method overloading feature, I have added two new type
definitions:

`action(name: string, func: T): T`
`action(func: T): T`

The existing `action()` type definition returned `any`, which caused
type information from passed wrapped functions to be lost. The new type
definitions return the same type of function that is passed as an
argument so that type information for the function is not lost when
wrapping the function in `action()`.

github.com/newoga

flow-typed/mobx.js

@@ -301,6 +301,8 @@ declare module 'mobx' {
   declare function extendShallowObservable(target: any): any;
 
   declare function action(targetOrName: any, propertyKeyOrFuc?: any, descriptor?: PropertyDescriptor): any;
+  declare function action<T>(name: string, func: T): T;
+  declare function action<T>(func: T): T;
   declare function runInAction<T>(name: string, block: () => T, scope?: any): T;
   declare function runInAction<T>(block: () => T, scope?: any): T;
   declare function isAction(thing: any): boolean;