refactor the velocity with a more scalable api and spi interfaces

[xiaoma20082008]

I want to refactor the velocity with a more scalable API (for Users) and SPI (for Developers), but I don't know whether it's needed, So I created this pr for ensure it (Please tell me if you need otherwise please close this pr, Thank you! ).

The target is:

1. compile the template(.vm) to a java class(.java), also provide a interpreter like now.
2. the template#render will compile the generated java code to a java class and execute it.

The sample usage For User:

Template template = VelocityEngine.getTemplate(name, locale, encoding);
template.render(context, writer);

Sample Implementation is:

Template getTemplate(String name, Locale locale, String encoding) {
  // 0. cache
  Resource r = cache.get(name);
  // 1. load (the loader maybe is ClasspathLoader, FileLoader, JarLoader... )
  r = loader.load(name, locale, encoding);
  // 2. read a .vm file to string with encoding
  String source = r.getSource();
  // 3. convert xxs characters
  source = converter.convert(name, source);
  // 4. compile
  Class<?> templateClass = compiler.compile(source); // maybe a CompiledTemplate or a InterpretedTemplate
  // 5. create template instance
  return templateClass.getConstructor().newInstance();
}

[xiaoma20082008]

Nice idea, but we won't introduce another logging abstraction. We have replaced log chutes with SLF4J for very good reasons. Drop that part please.

Got it, Thank you!

[AnthonyJClink]

This api looks alot like handlebars. this is pretty slick.