Enable and fix compiler warnings, add -Werror

[Totktonada]

The discussion was started here.

diff --git a/pom.xml b/pom.xml
index 67b6cb9..29bfc49 100644
--- a/pom.xml
+++ b/pom.xml
@@ -46,6 +46,9 @@
                 <configuration>
                     <source>1.6</source>
                     <target>1.6</target>
+                    <compilerArgs>
+                        <arg>-Xlint:all</arg>
+                        <arg>-Werror</arg>
+                    </compilerArgs>
                 </configuration>
             </plugin>