Add support for Java object macros

[kirsle]

This finally adds support for compile-time Java object macros to be used, bringing the API function setSubroutine() that most of the other implementations have.

New API Features

• void setSubroutine (String name, com.rivescript.ObjectMacro impl) -- to set an object macro written in Java at compile-time of your application.
• string currentUser () -- retrieve the current user's ID from within a Java object macro.
• string lastMatch (String username) -- retrieve the text of the last matched trigger for that user. The RSBot example bot calls this when you use the /last command.

Misc Changes

• Version numbering is switching to semver, so com.rivescript.RiveScript.VERSION is a string now like "0.6.0"

Object Macro Example

To implement an object macro written in Java you must implement the ObjectMacro class. Example:

public class MyMacro implements com.rivescript.ObjectMacro {
    public String call (com.rivescript.RiveScript rs, String[] args) {
        return "Hello, Java!";
    }
}

And register it using the setSubroutine() API function:

rs.setSubroutine("mine", new MyMacro());

And it's callable within RiveScript:

+ test my macro
- Here goes nothing: <call>mine</call>