Update .gitignore for CocoaPods

This removes 2 entries from the current `.gitignore`

1. The `Podfile.lock` (also checking in the current version). Without this file, if FMDB was updated to 3.0 with breaking changes, the next time someone cloned this and ran `pod install` they would install a broken version. Checking in this file ensures that pods will only be updated when running `pod update`. In the future you may want to add more specific information about acceptable versions as well. [Here's](http://guides.cocoapods.org/using/using-cocoapods.html#what-is-a-podfilelock) the relevant section of "Using CocoaPods" where we discuss this.
2. The generated `xcworkspace`. Since this project cannot be built without the dependencies installed via CocoaPods which go through this workspace, I'd recommend keeping this in the project. This will also show users who have just cloned the project that they are missing the `Pods.xcodeproj`.