Allow user to specify classLoader as loader property

PropertiesLauncher now supports creating its own class loader
from looader.classLoader property. It will succeed if the
implementation specified has a default constructor or one
that takes a parent class loader, or one that takes a URL[]
and a parent class loader (like URLClassLoader).
Add InitCommand to execute groovy script on startup

InitCommand runs on creation of SpringCli so it can search for additional
Commands in updated classpath. Also added as interactive command in Shell
session.
Enhance FileOptions to support multiple classpath resources
Extend programming model for script commands
First proper draft of DSL for Groovy Commands

Users can declare or Command, OptionHandler classes in an init script
or they can use a DSL, e.g.

command("foo") { args -> println "Do stuff with ${args} array" }

or

command("foo") {
  options { option "bar", "Help text for bar option" ithOptionArg() ofType Integer }
  run { options -> println "Do stuff with ${options.valueOf('bar')}" }
}