More YARDoc for RulesEngine#add_rule and #add_rules
Added a block syntax to define common conditions for adding rules

Example from the specs showcasing the #conditions method.

```
engine.add_rules 'Happy', true, :default do |rule|
  # Setup general conditions for the entire block
  rule.conditions rain: true do |rule|
    # Nobody likes rain...
    rule.variation false, :default
    # ...except for the Brits :-)
    rule.variation true, :law, country: 'GB'
    # ...or people with an umbrella
    rule.variation true, :law, umbrella: true
  end
end
```
Nesting conditions blocks merges conditions as expected