Implemented attributes for describing models

Note: this PR is based on #16

I implemented the attributes which can be used to describe models. An example:
```csharp
[Description("A person")]
public class TestPerson
{
    [Minimum(0)]
    [Maximum(150)]
    public int Age { get; set; }

    [UniqueItems]
    public IList<string> EmailAddresses { get; set; }

    [Name("firstname")]
    [Description("Person's first name")]
    [Required]
    public string FirstName { get; set; }

    [Enum("male", "female")]
    public string Gender { get; set; }
}
```

@liddellj, @khellang:  I chose not implementing an <code>ISwaggerModelDataProvider</code> and <code>ISwaggerModelCatalog</code>, but let the SwaggerAnnotationsConverter discover the model data itself. 
When it is needed, the code can easily be moved to provider/catalog class, but at this point doing it felt a bit like over-designing.