Fixed compilation problems in QSDatabaseQueue

Ran into a couple of problems while trying to compile QSDatabaseQueue.m in a project that used Xcode 5, ARC, and FMDB 2.1.

1. `QSDatabaseQueue` didn't import `QSPlatform.h` which was generating a compilation error in the `initWithFilename:excludeFromBackup:` method: 

    **Implicit conversion of 'int' to 'NSString*' is disallowed with ARC**

2. I couldn't find the `FMDatabase+Extras.h` header file in any recent version of FMDB nor did it seem to be included in QSKit

3. The `FMDatabase` class doesn't seem to declare an `openDatabaseWithPath:` class method. I took a stab at adding it to `QSDatabaseQueue`, though I'll admit that I didn't actually test my implementation. I'm assuming this was likely one of the methods declared in the missing `FMDatabase+Extras.h` header file