added missing static functions

Function definition for Dexie.delete at Dexie.js:3246
Function definition for Dexie.exists at Dexie.js:3259

Other static definitions are still missing.

dexie/dexie.d.ts

@@ -37,6 +37,10 @@ declare class Dexie {
     static shallowClone(obj: Object): Object;
 
     static deepClone(obj: Object): Object;
+    
+    static delete(databaseName : string): Dexie.Promise<void>;
+    
+    static exists(databaseName : string): Dexie.Promise<bool>;
 
     version(versionNumber: number): Dexie.Version;