Make active_decorator.rb independent from Rails

Like kaminari,
https://github.com/amatsuda/kaminari/blob/master/lib/kaminari.rb
Work with Sinatra

To enable ActiveDecorator in Sinatra, register it like:

```ruby
require "active_decorator/sinatra"
register ActiveDecorator::Sinatra
```

Then ActiveDecorator::Sinatra reads `ROOT/decorators/*_decorator.rb`,
and decorates instance variables in `scope` and local variables passed
via `render` method with them.

The `ROOT/decorators` directory is used by default. If you want to use
another directory, you can set it as `:decorators` like:

```ruby
require "active_decorator/sinatra"
set :decorators, "/path/to/your/decorators
register ActiveDecorator::Sinatra
```

ActiveDecorator::Sinatra doesn't decorate following instance variables
by default:

```
@default_layout @env @params @preferred_extension @request
@response @template_cache
```

These are declared in an array named as
`ActiveDecorator::Sinatra::DEFAULT_PROTECTED_INSTANCE_VARIABLES`. If you
want to protect other instance variables, you can declare them as
`:protected_instance_variables` like:

```ruby
require "active_decorator/sinatra"
set :protected_instance_variables, %w[@foo @bar]
register ActiveDecorator::Sinatra
```

Note that these `set` statements must be placed before the `register`
statement.