Defined PullParameter#defaultValue as []

This allows to declare empty strings as default values: {""}; additional
checks occurs @runtime
inject MojoExecution
Use ExpressionEvaluator in ConfigHandler to get default values
get the default value of proguard outputDirectory from @PullParameter
Skip proguard execution if the configuration file is missing; it allows
to set android.proguard.skip=false, but only project having a
configuration file will be obfuscated
Give info about reasons of skipping Proguard.
Merge pull request #259 from pa314159/pull/config-handler

config handler improvments