fixes JOO-69
Maven compile goals bypass Maven log
http://jangaroo.myjetbrains.com/youtrack/issue/JOO-69

Calling AbstractMojo#getLog() in a field initializer leads to
strange effects, because the "real" log has not yet been injected.
Additionally, our AbstractCompilerMojo sometimes used its
"log" field and sometimes getLog(). This explains the weird behavior.