Reorganized directory structure a little

Pursuant to [issue 234](https://github.com/ccgus/fmdb/issues/234), I have:

- reorganized the folder structure of the project, moving FM*.{h,m} files from `src` into `src/fmdb` (to make it easier for people who aren't using CocoaPods, but rather are copying the source into their projects);

- renamed `fmdb.m` to `main.m` and moved it into `src/sample`, and also moved the `.pch` to `src/sample`, too;

- moved `extras` to `src/extras`; and

- created new `FMDB.h` file that does the `#import` of all the main fmdb header files for people who would like to just add one `#import` to their projects.

I must confess that I'm not familiar with CocoaPods `podspec` intricacies, so I really need someone to take a look at the `fmdb.podspec` to make sure everything is copacetic there (e.g. at the very least, the `exclude_files` now seems redundant, but I wouldn't be surprised if other changes were necessary, too).