Expose full-text module (FTS3) as optional extra 

This pull request contains a category for `FMDatabase` and `FMResultSet` that allows FMDB users to work with the full-text index module in SQLite. It has the following features:
1) Implement custom tokenizers in Objective-C via a custom protocol. The library includes two tokenizers. The "simple" tokenizer supports localized, Unicode-aware tokenization via `CFStringTokenizer`. The other tokenizer provides support for a "stop words" list, which is used to exclude certain terms from the index.
2) Make working with the `offsets()` function easier.
3) Provide an interface to execute FTS3/4 "special commands".

The interfaces and protocols have some documentation in the comments, but I plan on providing an additional README with more details.

I think this will be useful for Cocoa developers that want to add full-text search to their applications.