add ability to clear store

spec/database.spec.ts

@@ -188,4 +188,29 @@ describe('database functionality', () => {
       });
   });
 
+  it('should clear store', (done) => {
+    let found;
+    idb.clear('todos').subscribe(
+      () => {},
+      err => {
+        console.error(err),
+        done(err);
+      },
+      () => {
+        idb.query('todos').toArray().subscribe(
+          (records) => {
+            found = records;
+          },
+          err => {
+            console.error(err);
+            done(err);
+          },
+          () => {
+            expect(found.length).toEqual(0);
+            done();
+          }
+        );
+      })
+  });
+
 });

src/database.ts

@@ -269,6 +269,38 @@ export class Database {
   compare(a: any, b: any): number {
     return this._idb.cmp(a, b);
   }
+
+  clear(storeName: string) {
+    const open$ = this.open(this._schema.name);
+    return mergeMap.call(open$, (db: IDBDatabase) => {
+        return new Observable( (txnObserver: Observer<any>) => {
+         const recordSchema = this._schema.stores[storeName];
+         const mapper = this._mapRecord(recordSchema);
+         const txn = db.transaction([storeName], IDB_TXN_READWRITE);
+         const objectStore = txn.objectStore(storeName);
+
+         const clearRequest = objectStore.clear();
+
+         const onTxnError = (err: any) => txnObserver.error(err);
+         const onTxnComplete = () => txnObserver.complete();
+         const onClear = () => txnObserver.next(null);
+
+         txn.addEventListener(IDB_COMPLETE, onTxnComplete);
+         txn.addEventListener(IDB_ERROR, onTxnError);
+
+         clearRequest.addEventListener(IDB_SUCCESS, onClear);
+         clearRequest.addEventListener(IDB_ERROR, onTxnError);
+
+         return () => {
+           clearRequest.removeEventListener(IDB_SUCCESS, onClear);
+           clearRequest.removeEventListener(IDB_ERROR, onTxnError);
+           txn.removeEventListener(IDB_COMPLETE, onTxnComplete);
+           txn.removeEventListener(IDB_ERROR, onTxnError);
+         };
+
+        });
+      });
+  }
 }