Fixed classloader issue in TestMojo.

* Created new classloader composing transitive dependencies.
* Fixed webDriverClassName instantiation due classloader issue.
Fixed issue #96: specs aren't executed in the headless run.

* Copy sources and specs to the target directory since runners read from it.
Added support to define system properties in the plugin configuration.

* Map configured system properties into System#properties collection.
Merge branches 'bugs/96_specs_in_headless_run', 'bugs/113_testmojo_classloader' and 'features/system_properties'