Merge pull request #221 from FrankSalad/v3-exists-fn

V3 exists fn

Author: auser

Diff for: docs/api/database.md

@@ -25,6 +25,15 @@ firestack.database()
   });
 ```
 
+Test value exists at location:
+```javascript
+firestack.database()
+  .ref('posts/1234')
+  .on('value', (snapshot) => {
+    const exists = snapshot.exists();
+  });
+```
+
 Basic write with priority example:
 ```javascript
 firestack.database()

Diff for: lib/modules/database/snapshot.js

@@ -22,7 +22,6 @@ export default class Snapshot {
     this.ref = ref;
     this.key = snapshot.key;
     this.value = snapshot.value;
-    this.exists = snapshot.exists || true;
     this.priority = snapshot.priority === undefined ? null : snapshot.priority;
     this.childKeys = snapshot.childKeys || [];
   }