Introducing 'lotus/setup'

This is a file to be required to have all the Lotus libraries loaded and Bundler groups required.

## Use case

```ruby
# Gemfile
gem 'bundler'
gem 'lotusrb'

group :test do
  gem 'capybara'
end

group :production do
  gem 'puma'
end
```

```ruby
# config/environment.rb
require 'lotus/setup'
```

Requiring that file will cause:

  * Require Rubygems (`require 'rubygems'`)
  * Require and setup Bundler's group (`require 'bundler/setup'`). This requires `lotusrb` from the example above)
  * Require Bundler's extra groups. This lookups the current environment (`ENV['LOTUS_ENV']` or `ENV['RACK_ENV']`) to determine what to load. In the example above, `capybara` will be loaded if the current env is equal to `test`, or `puma` in case of `production`.
Why is this needed?