Customizable rules

Discussed in #8

This at the moment already works, but still needs a bit more work.

This is the default config file:
```yaml
---
threshold: 100
rules:
  ApiTag::Presence:
    enabled: true
    exclude: []
  ApiTag::Inclusion:
    enabled: true
    exclude: []
  ApiTag::ProtectedMethod:
    enabled: true
    exclude: []
  ApiTag::PrivateMethod:
    enabled: true
    exclude: []
  ExampleTag:
    enabled: true
    exclude: []
  ReturnTag:
    enabled: true
    exclude: []
  Summary::Presence:
    enabled: true
    exclude: []
  Summary::Length:
    enabled: true
    exclude: []
  Summary::Delimiter:
    enabled: true
    exclude: []
  Summary::SingleLine:
    enabled: true
    exclude: []
```

It can be used like this:
```ruby
  config = YAML.load_file(File.expand_path('../../../config/yardstick.yml', __FILE__))
  Yardstick::Rake::Verify.new(config)
```

I am interested to know if you don't like something or want something to be changed.

Also, I followed the DM2 coding style and have only tested it with ruby 1.9.3.