Build: Specifying java source version in pom

- This makes is easier to import pom into InteliJ.

pom.xml

@@ -44,6 +44,13 @@
         <sourceDirectory>src</sourceDirectory>
         <testSourceDirectory>test</testSourceDirectory>
         <plugins>
+            <plugin>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>1.7</source>
+                    <target>1.7</target>
+                </configuration>
+            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-source-plugin</artifactId>