Replace Javadoc annotations by Java annotations.

@see http://maven.apache.org/plugin-tools/maven-plugin-plugin/examples/using-annotations.html
Keep cause when throwing MojoExecutionException.
Set a property to define 'packages' parameter: 'assertj.packages'.

It can now be set by using: mvn assertj-generate-assertions -Dassertj.packages=your.first.package,your.second.package
Replace 'TARGET_DIR' by a temporary folder managed by JUnit rule.
Add parameter 'classes' as it is supported by the assertions generator.
Set a property to define 'targetDir' parameter: 'assertj.targetDir'.
Fail the build if parameter 'packages' or 'classes' are both null or empty.
Generate Help Mojo accessible via 'mvn assertj:help'.
Merge branch 'java-annotations' of github.com:jcgay/assertj-assertions-generator-maven-plugin into jcgay-java-annotations

Conflicts:
	src/main/java/org/assertj/maven/AssertJAssertionsGeneratorMojo.java
	src/main/java/org/assertj/maven/generator/AssertionsGenerator.java
	src/test/java/org/assertj/maven/AssertJAssertionsGeneratorMojoTest.java