Fix up a few details: TrafficSource and database.DeltaSnapshot.toJSON() docs. (#134)

Fix up a few details pointed out by jwenger@:
- The TrafficSource interface was still present, and unused.
- The database.DeltaSnaphsot.toJSON() documentation was a little confusing to people who aren't familiar with the DeltaSnapshot concepts of previous/current.
- child's path argument isn't optional in the client SDK's DataSnapshot, so that specification just adds confusion to the interface. We'll still handle undefined/null cases gracefully, just in case.

Author: Robert-Jan Huijsman

src/providers/analytics.ts

@@ -335,18 +335,6 @@ export interface AppInfo {
   appId?: string;
 }
 
-/** Information about the marketing campaign which acquired the user that triggered these events. */
-export interface TrafficSource {
-  /** The name of the network which acquired the user. Eg. "Google". */
-  userAcquiredSource?: string;
-
-  /** The name of the medium which acquired the user. Eg. "Banner". */
-  userAcquiredMedium?: string;
-
-  /** The name of the campaign which acquired the user. Eg. "Winter Promo". */
-  userAcquiredCampaign?: string;
-}
-
 /**
  * Interface representing the bundle in which these events were uploaded.
  */

src/providers/database.ts

@@ -187,7 +187,7 @@ export class DeltaSnapshot implements firebase.database.DataSnapshot {
     return !_.isNull(this.val());
   }
 
-  child(childPath?: string): DeltaSnapshot {
+  child(childPath: string): DeltaSnapshot {
     if (!childPath) {
       return this;
     }
@@ -229,8 +229,9 @@ export class DeltaSnapshot implements firebase.database.DataSnapshot {
     return _.isPlainObject(val) ? Object.keys(val).length : 0;
   }
 
-  /** Prints the value of the snapshot; use '.previous' and '.current' to explicitly see
-   *  the previous and current values.
+  /**
+   * Prints the value of the snapshot; use '.previous.toJSON()' and '.current.toJSON()' to explicitly see
+   * the previous and current values of the snapshot.
    */
   toJSON(): Object {
     return this.val();