Merge pull request #17 from sgrossberndt/master

Display installedPiwikNeeded only if not installed
Handle unsuccessful removePiwik with error message

Throw an exception if piwik directory could not be deleted.
catch exceptions from methods with flash error message instead of displaying ok message
Merge pull request #18 from sgrossberndt/patch-1

Handle unsuccessful removePiwik with error message
Localized messages for extension update wizard