Add Checkstyle for Trailing Whitespace (#4497)

Author: vanniktech

build.gradle

@@ -1,7 +1,7 @@
 buildscript {
   repositories { jcenter() }
-  dependencies { 
-    classpath 'com.netflix.nebula:gradle-rxjava-project-plugin:4.0.0' 
+  dependencies {
+    classpath 'com.netflix.nebula:gradle-rxjava-project-plugin:4.0.0'
     classpath 'ru.vyarus:gradle-animalsniffer-plugin:1.1.0'
   }
 }
@@ -13,6 +13,7 @@ description = 'RxJava: Reactive Extensions for the JVM – a library for composi
 apply plugin: 'java'
 // apply plugin: 'pmd'
 apply plugin: 'findbugs'
+apply plugin: 'checkstyle'
 apply plugin: 'jacoco'
 apply plugin: 'ru.vyarus.animalsniffer'
 apply plugin: 'nebula.rxjava-project'
@@ -120,6 +121,10 @@ build.dependsOn jacocoTestReport
 
 // build.dependsOn pmdPrint
 
+checkstyle {
+    configFile file('checkstyle.xml')
+}
+
 findbugs {
     ignoreFailures true
     toolVersion = '3.0.1'

checkstyle.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+    "-//Puppy Crawl//DTD Check Configuration 1.2//EN"
+    "http://www.puppycrawl.com/dtds/configuration_1_2.dtd">
+<module name="Checker">
+    <module name="TreeWalker">
+        <module name="RegexpSinglelineJava">
+            <property name="format" value="^(?!\s+\* $).*?\s+$"/>
+            <property name="message" value="Line has trailing spaces."/>
+        </module>
+    </module>
+</module>