Return a list of all extensions from a plugin and optional for an extension point

[decebals]

In few words, this PR add below methods in PluginManager:
- <T> List<T> getExtensions(Class<T> type, String pluginId)
- List getExtensions(String pluginId)

See more details about these methods on https://groups.google.com/forum/#!topic/pf4j/vqnD1abLXjA.

[coveralls]

Coverage decreased (-0.7%) to 13.639% when pulling f595eb5 on extensions_per_plugin into d6f9ccb on master.

[coveralls]

Coverage decreased (-0.7%) to 13.631% when pulling 0e21831 on extensions_per_plugin into d6f9ccb on master.

[coveralls]

Coverage decreased (-0.7%) to 13.631% when pulling 0e21831 on extensions_per_plugin into d6f9ccb on master.