Allow validateSession to return a promise instead of an immediate value (#36)

* Allow promise like object/function to be provided as validateSession function
Extracted code that would become duplicated since the promise addition

* Handle promised validateSession return value the same as if it was a function that would return an immediate value.

* Fixed lint issues

* Added tests for validateSession function

* Fixed readme to specify that the validateSession can be a function that returns a promise.

* Added code example for a validateSession that returns a promise.

Author: darkrift

README.md

@@ -67,8 +67,7 @@ Options:
 - refreshOnCheckAuth(**default**: false): Refresh Redux store in the `checkAuth` function
 - redirectPath(**default**: `"login"`): Path used when a session is rejected or doesn't exist
 - driver: Force to use a particular driver, could be: 'INDEXEDDB', 'WEBSQL', 'LOCALSTORAGE' or 'COOKIES'
-- validateSession: Function to validate the saved session, it should return a BOOLEAN. If it returns `false` the session will be destroyed
-
+- validateSession: Function to validate the saved session. It can either be a function to return an immediate boolean value or a function that returns a promise. In the case it returns an immadiate value and `false` is returned the session will be destroyed. In the case of a promise, if either `false` is returned or an exception is thrown, the session will be destroyed.
 Example:
 ```javascript
 const validateSession = (session) => {
@@ -82,6 +81,18 @@ sessionService.initSessionService(store, options)
   .catch(() => console.log('Redux React Session is ready and there is no session in your storage'));
 ```
 
+```javascript
+const validateSession = (session) => {
+  // check if your session is still valid with a server check, through axios for instance
+  return api.invokeRemoteSessionValidationThroughAxios(session).then(response => response.isSessionValid);
+}
+const options = { refreshOnCheckAuth: true, redirectPath: '/home', driver: 'COOKIES', validateSession };
+
+sessionService.initSessionService(store, options)
+  .then(() => console.log('Redux React Session is ready and a session was refreshed from your storage'))
+  .catch(() => console.log('Redux React Session is ready and there is no session in your storage'));
+```
+
 ### refreshFromLocalStorage
 Force to refresh the Redux Store from the local storage.