Breaking changes

Many breaking changes from 0.3 to 0.4:

• Changed some class names / namespaces
• Removed JS files in favor of a dedicated standalone component webpush-client
• Ajax payload now expects {"subscription": {Subscription object}, "options": {Options object}"}
• Added UserSubscriptionInterface::getContentEncoding() method
• Changed UserSubscriptionManagerInterface::hash() signature
• Removed Notification value object in favor of PushNotification
• Changed bundle configuration
• New PushMessageSender class.
• Added tests + travis (still need some more coverage)