Add JarPluginManager, PluginLoader, AbstractPluginManager

[decebals]

This PR comes with many good things.
First of all, it comes with a builtin JarPluginManager (see #118). According javadoc:

It's a {@link PluginManager} that load plugin from a jar file.
Actually, a plugin is a fat jar, a jar which contains classes from all the libraries,
on which your project depends and, of course, the classes of current project.

So, your plugins are jar files that could be created using Maven Shade Plugin.

To add the possibility to create custom PluginManager more easy, this PR introduces new concepts:

• AbstractPluginManager
• PluginLoader

The current implementation of JarPluginManager is:

public class JarPluginManager extends DefaultPluginManager {

    @Override
    protected PluginRepository createPluginRepository() {
        return new JarPluginRepository(getPluginsRoot(), isDevelopment());
    }

    @Override
    protected PluginDescriptorFinder createPluginDescriptorFinder() {
        return isDevelopment() ? new PropertiesPluginDescriptorFinder() : new JarPluginDescriptorFinder();
    }

    @Override
    protected PluginLoader createPluginLoader() {
        return new JarPluginLoader(this, pluginClasspath);
    }

}

[coveralls]

Coverage increased (+1.8%) to 15.176% when pulling d7f245e on jar_plugin_manager into c82df37 on master.