Enforce semantic versioning

[fnothaft]

We'd probably want to hold off on this until 1.0 or so, but Parquet uses the maven enforcer plugin to flag API breaking changes:

        <plugin>
          <artifactId>maven-enforcer-plugin</artifactId>
          <version>1.3.1</version>
          <dependencies>
            <dependency>
              <groupId>org.semver</groupId>
              <artifactId>enforcer-rule</artifactId>
              <version>0.9.23</version>
            </dependency>
         </dependencies>
         <executions>
           <execution>
             <id>check</id>
             <phase>verify</phase>
             <goals>
               <goal>enforce</goal>
             </goals>
             <configuration>
               <rules>
                 <requireBackwardCompatibility implementation="org.semver.enforcer.RequireSemanticVersioningConformance">
                   <dumpDetails>true</dumpDetails>
                   <previousVersion>${previous.version}</previousVersion>
                   <excludes>
                     <exclude>parquet/org/**</exclude>
                   </excludes>
                 </requireBackwardCompatibility>
               </rules>
             </configuration>
           </execution>
         </executions>
        </plugin>

[tdanford]

+1

[heuermh]

Semantic versioning is now in place as of release version 1.0.