Make input/output dir customizable. Allows simpler eclipse setups.

You can now invoke the plugin with 

```xml
<plugin>
				<groupId>com.github.drochetti</groupId>
				<artifactId>javassist-maven-plugin</artifactId>
				<version>1.0.1-SNAPSHOT</version>
				<configuration>
					<includeTestClasses>false</includeTestClasses>
                                        <-- This option is new -->
                                        <buildDir>bin/classes</buildDir>
					<transformerClasses>
						<transformerClass>
							<className>afterburner.PostProcessor</className>
						</transformerClass>
					</transformerClasses>
				</configuration>
				<executions>
					<execution>
						<phase>process-classes</phase>
						<goals>
							<goal>javassist</goal>
						</goals>
					</execution>
				</executions>
			</plugin>
```

or by command line, for instance when creating an eclipse builder : 

```bash
mvn com.github.drochetti:javassist-maven-plugin:javassist -DbuildDir=bin/classes/
```