Add checkstyle support

Fixes #481

Execute checkstyle validation at build time.
Add minimal set of rules.
Ensure that existing code conforms to the rules.

src/checkstyle/checkstyle.xml

@@ -0,0 +1,21 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+		"-//Puppy Crawl//DTD Check Configuration 1.3//EN"
+		"http://www.puppycrawl.com/dtds/configuration_1_3.dtd">
+<module name="Checker">
+	<module name="TreeWalker">
+		<!-- this. in front of fields -->
+		<module name="RequireThis">
+			<property name="checkMethods" value="false"/>
+		</module>
+		<!-- tabs instead of spaces -->
+		<module name="RegexpSinglelineJava">
+			<property name="format" value="^\t* "/>
+			<property name="message" value="Indent must use tab characters"/>
+			<property name="ignoreComments" value="true"/>
+		</module>
+		<module name="UnusedImports"/>
+		<module name="RedundantImport"/>
+		<module name="NeedBraces"/>
+	</module>
+</module>