Run against ActiveRecord's tests

I was able to get Firebird to run against ActiveRecord's tests. These changes were inspired by [rails-sqlserver/activerecord-sqlserver-adapter](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter).

Unfortunately, a few minor changes need to be made in the ActiveRecord codebase in order to get this to work (see [the diff](https://github.com/rzane/rails/compare/firebird-support)). I've merged those changes into my own [4-0-stable](https://github.com/rzane/rails/tree/4-0-stable) and [4-1-stable](https://github.com/rzane/rails/tree/4-1-stable) branches, so that we can run the tests for now. Hopefully, we can get those changes merged into Rails. Until then, maybe this PR should stay open?

To run the tests:
```
git clone git@github.com:rzane/rails -b 4-0-stable
git clone git@github.com:rzane/activerecord-fb-adapter -b update-tests
cd activerecord-fb-adapter
export RAILS_SOURCE=../rails
bundle install
mkdir db && chmod o+w db
bundle exec rake
```

I also made a few minor changes to the adapter:

+ Made `create_sequence` and `drop_sequence` public methods.
+ Got already existing tests passing and moved table setup to `fb_specific_schema.rb`.
+ Bumped FB version in the gemspec and added AR 4.0 as a dependency.
+ Replaced hyphens in generator names with underscores.
+ Updated the README because I've been using this library on OSX for about a year.