Add source encoding to the maven compiler plugin.

This change adds the source encoding to the maven compiler plugin, which
will strictly enforce build consistency on multiple platforms and address
possible compilation issues on some of the source files. The source
encoding unintuitively is system-specified by default.

By: Wesley Wolfe <weswolf@aol.com>

paper-api/pom.xml

@@ -47,6 +47,7 @@
                 <configuration>
                     <source>1.6</source>
                     <target>1.6</target>
+                    <encoding>${project.build.sourceEncoding}</encoding>
                 </configuration>
             </plugin>
             <plugin>