add signInWithProvider with google, facebook

Author: choru-k

README.md

@@ -250,6 +250,31 @@ authManager.authorizeWithCallbackURL('twitter', appUrl)
 })
 ```
 
+### socialLogin with custom Library
+If you don't want to use [react-native-oauth](https://github.com/fullstackreact/react-native-oauth), you can use other library such as [react-native-facebook-login](https://github.com/magus/react-native-facebook-login). 
+
+```javascript
+var {FBLogin, FBLoginManager} = require('react-native-facebook-login');
+
+var Login = React.createClass({
+  render: function() {
+    return (
+      <FBLogin 
+        onLogin={function(data){
+          console.log("Logged in!");
+          console.log(data);
+          let token = data.credentials.token
+          firestack.signInWithProvider('facebook', token, '') // facebook need only access token.
+            .then((user)=>{
+              console.log(user)
+            })
+        }}
+      />
+    );
+  }
+});
+```
+
 If the `signInWithProvider()` method resolves correct and we have already set up our `listenForAuth()` method properly, it will fire and we'll have a logged in user through Firebase.
 
 ### reauthenticateWithCredentialForProvider()

ios/Firestack/Firestack.m

@@ -619,6 +619,11 @@ - (FIRAuthCredential *)getCredentialForProvider:(NSString *)provider
     if ([provider isEqualToString: @"twitter"]) {
         credential = [FIRTwitterAuthProvider credentialWithToken:authToken
                                                           secret:authTokenSecret];
+    } if ([provider isEqualToString: @"facebook"]) {
+        credential = [FIRFacebookAuthProvider credentialWithAccessToken:authToken];
+    } if ([provider isEqualToString: @"google"]) {
+        credential = [FIRGoogleAuthProvider credentialWithToken:authToken
+                                                          secret:authTokenSecret];
     } else {
         NSLog(@"Provider not yet handled");
     }