Added ResourcedOptionAttribute.

This allows users to move all option help text into resources. I experimented a while with getting a ```ResourceManager``` from ```Assembly.GetEntryAssembly()```, but it was tricky to get generic enough. And explicit is better than implicit.

Replaces issue #20. 

Here's an example on how to use it:

```
class Program
{
	private class Options : CommandLineOptionsBase
	{
		public Options()
		{
			// Could be replaced with some logic
			// selecting other (localized) resources
			ResourcedOptionAttribute.ResourceManager =
				Properties.Resource1.ResourceManager;
		}

		[ResourcedOption('f', "foo", Required = true,
			HelpTextKey = "FooHelpText")]
		public string Foo { get; set; }

		[HelpOption]
		public string GetUsage()
		{
			return HelpText.AutoBuild(this, current =>
				HelpText.DefaultParsingErrorsHandler(this, current));
		}
	}

	static void Main(string[] args)
	{
		var options = new Options();
		if (!CommandLineParser.Default.ParseArguments(args, options))
		{
			Environment.Exit(-1);
		}
	}
}
```