Consolidate java-templates folder into rest of module (#174)

* - Consolidate java-templates folder into rest of module
- Mark immutable variables as final
- Remove unused `apiVersion` global variable

* - Undo move, template file needed to generate VERSION variable dynamically from pom.xml

* - Clarify how Maven template variable work

Author: nwithan8

src/main/java-templates/com/easypost/EasyPost.java

@@ -2,12 +2,37 @@
 
 import com.easypost.easyvcr.VCR;
 
+/**
+ * Hello weary traveler, welcome to the EasyPost Java client library.
+ * <p>
+ * This file exists as a template for the Templating Maven Plugin (https://www.mojohaus.org/templating-maven-plugin/)
+ * If you notice the VERSION = ${project.version} below, that's an example of a template variable.
+ * <p>
+ * In Maven (and Gradle), you can set variables inside the pom.xml file
+ * (https://maven.apache.org/guides/introduction/introduction-to-the-pom.html#available-variables)
+ * <p>
+ * The Templating Maven Plugin, at compile time, will extract these variables and add them into the source code.
+ * <p>
+ * Specifically here, VERSION = ${project.version} will be replaced with, i.e. VERSION = 1.0.0 when compiling the code.
+ * <p>
+ * The placement of this file is important.
+ * The Templating Maven Plugin will look for any template files in this specific `java-templates` directory.
+ * As a result, you can not remove this file.
+ * (https://www.hascode.com/2013/09/filtering-source-files-using-the-templating-maven-plugin/)
+ * <p>
+ * Most of the other variables in this file are not template variables.
+ * Instead, we simply use this file as a catch-all for all global variables.
+ * <p>
+ * If you ever run across a section of code where your IDE says it cannot find, i.e. `EasyPost.apiKey`,
+ * it's likely because this file has not been compiled yet.
+ * Simply run `mvn install` (or `make build-dev`) to compile this code and your IDE will be able to find the variable.
+ */
+
 public abstract class EasyPost {
     public static final String VERSION = "${project.version}";
-    public static String API_BASE = "https://api.easypost.com/v2";
-    public static String BETA_API_BASE = "https://api.easypost.com/beta";
+    public static final String API_BASE = "https://api.easypost.com/v2";
+    public static final String BETA_API_BASE = "https://api.easypost.com/beta";
     public static String apiKey;
-    public static String apiVersion;
     public static int readTimeout;
 
     /**