implmented milestone api via v3

src/main/java/org/kohsuke/github/GHMilestone.java

@@ -0,0 +1,72 @@
+package org.kohsuke.github;
+
+import java.util.Date;
+
+/**
+ * 
+ * @author Yusuke Kokubo
+ *
+ */
+public class GHMilestone {
+    GitHub root;
+	GHRepository owner;
+
+	GHUser creator;
+	private String state, due_on, title, url, created_at, description;
+	private int closed_issues, open_issues, number;
+
+	public GitHub getRoot() {
+		return root;
+	}
+	
+	public GHRepository getOwner() {
+		return owner;
+	}
+	
+	public GHUser getCreator() {
+		return creator;
+	}
+	
+	public Date getDueOn() {
+		if (due_on == null) return null;
+		return GitHub.parseDate(due_on);
+	}
+	
+	public String getTitle() {
+		return title;
+	}
+	
+	public String getUrl() {
+		return url;
+	}
+	
+	public Date getCreatedAt() {
+		return GitHub.parseDate(created_at);
+	}
+	
+	public String getDescription() {
+		return description;
+	}
+	
+	public int getClosedIssues() {
+		return closed_issues;
+	}
+	
+	public int getOpenIssues() {
+		return open_issues;
+	}
+	
+	public int getNumber() {
+		return number;
+	}
+	
+	public GHMilestoneState getState() {
+		return Enum.valueOf(GHMilestoneState.class, state);
+	}
+
+	public GHMilestone wrap(GHRepository repo) {
+		this.owner = repo;
+		this.root = repo.root;
+		return this;
+	}
+}